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This is a major release, introducing many new features and
modifications. Most importantly, libplacebo now interoperates well with
FFmpeg's libav* abstractions. This is primarily exposed via a new set of
helpers, <libplacebo/utils/libav.h>, implemented as a single header
library. In addition to this, a number of other supporting changes have
been made to the API, most notably the unification of `pl_image` and
`pl_render_target` into a single `pl_frame` concept, similar in spirit
to AVFrame. As such, libplacebo now supports **rendering to planar
targets**, including subsampled YCbCr.

Besides the libav* compatibility changes, this release also brings with
it a new feature for custom shaders: buffer blocks, and persistent
storage. This can be used by third parties to implement stateful shaders
(e.g. motion interpolation or temporal deinterlacing), or be leveraged
to speed up some shaders by combining multiple passes into one.

Finally, various import/export procedures have been expanded, including
the ability to import host pointers and real-world DMABUFs.

- add `pl_memory_qualifiers`, plus a corresponding `pl_shader_desc.memory`,
  to allow attaching GLSL memory qualifiers (coherent, volatile etc.) to
  shader descriptors
- add functions `pl_dispatch_save` and `pl_dispatch_load` to allow
  saving/restoring the contents of an entire `pl_dispatch`'s cache
- add functions `pl_renderer_save` and `pl_renderer_load` to allow
  saving/restoring the contents of an entire `pl_renderer`'s cache
- add `pl_vulkan_swapchain_params.prefer_hdr`, which will cause the
  surface format selection logic to try HDR output formats first
- add `pl_buf_copy` to copy from one buffer to another
- add `pl_get_detected_peak`, to read back the result of peak detection
- add `pl_primaries_superset` to test if one set of primaries is fully
  enclosed by another
- add `pl_color_map_params.gamut_clipping`, which will colorimetrically
  clip any out-of-gamut colors by desaturating them towards neutral gray
  until they're in-gamut, rather than clipping per channel as before
- add `PL_GPU_CAP_SUBGROUPS` and `pl_gpu_limits.subgroup_size`, to
  expose GLSL subgroup functionality via the `pl_gpu` interface
- add `pl_gpu_is_failed`, to query at a high level whether the `pl_gpu`
  is in some internal failure state. GPUs in this state should be
  recreated, using the appropriate mechanism
- add `pl_shader_custom`, to allow injecting arbitrary custom GLSL code
  into a `pl_shader`.
- add `pl_buf_params.import_handle` to allow importing buffers
- add `PL_HANDLE_HOST_POTR`, to allow importing arbitrary host pointers
- add `pl_pass_run_params.vertex_buf`, to allow drawing vertex data
  directly from a `pl_buf`, guarded by `pl_gpu_limits.max_vbo_size`
- add `_COUNT` members to all public enums, for consistency
- add `pl_shared_mem.drm_format_mod`, to allow communicating DRM format
  modifiers when importing/exporting textures
- add support for importing DMABUFs via EGL, via the new fields
- add `pl_fmt.fourcc` to facilitate mapping between `pl_fmt` and DRM
- add the missing `pl_var_*` helpers, for consistency
- add `pl_plane_data_align` to help with aligning `pl_plane_data`
  structs to byte boundaries
- add support for STORAGE textures in user shaders, which can be used to
  persist data across separate invocations of the shader
- add support for BUFFER blocks in user shaders, which can be used to
  create UBOs or SSBOs for use inside shaders, the latter of which can
  also persist across frames and be used to store persistent state
- add a new header <libplacebo/utils/libav.h>, containing a variety of
  helper functions for interoperating between libav* and libplacebo
- add `demos/plplay.c` to serve as a demonstration of how to make a
  trivial playback loop with libavcodec and libplacebo
- add `pl_sample_src.component_mask` to allow sampling an arbitrary
  subset of the available components from a plane
- add `pl_frame_is_cropped` and `pl_frame_clear` to assist in properly
  clearing frames before rendering to them
- add `pl_tex_poll` to assist in interoperating with some external APIs
- add `pl_render_params.blend_params` to allow blending the final output

- remove `pl_image.signature` and `pl_render_params.skip_redraw_caching`
- change vulkan surface format selection to prioritize formats by
  'score', preferring higher depth integer formats
- `pl_fmt` may now have PL_FMT_CAP_STORABLE even when `glsl_format` is
  NULL, in which case formatless image storage must be used
- `pl_buf_read` no longer requires `buf_offset` be a multiple of 4
- `pl_buf_*` commands are now synchronized internally:
- `pl_buf_write` and `pl_buf_read` now block while the buffer is in use
  Note: for this reason, `pl_buf_write` should not be used in loops
- `pl_tex_upload/download` may now be called on in-use buffers
- allow `pl_dispatch_compute` on shaders with outputs, including the
  ability to automatically determine the number of work groups based on
  the shader output resolution
- remove `pl_buf_params.type`, and the concept of buffer types in
  general. `pl_buf` is now a generic catch-all for any type of buffer,
  with individual capabilities in `pl_buf_params` determinig what type
  of shader operations it can be used for
- relax the alignment requirements on `pl_tex_transfer_params`
- change `pl_opengl_wrap_params` to allow directly importing
  framebuffers in addition to textures
- rename `pl_color_levels` members for clarity
- make `pl_opengl` ignore software rasterizers by default, unless the
  new field `pl_opengl_params.allow_software` is set
- add `pl_av1_grain_params.luma_comp` to allow drawing the luma
  component from a channel with nonzero index
- `pl_renderer` now supports rendering to planar targets, including
  subsampled targets
- `pl_image` and `pl_render_target` have been removed and unified into a
  single `pl_frame` concept
- remove `pl_tex_params.sample_mode/address_mode` and move them to
  `pl_desc_binding` instead, to decouple them from texture creation
- refactor the signature of `pl_tex_blit`

Fixes and performance improvements:
- fix an oversight where `pl_buf_destroy` delayed some buffer
  destructions unnecessarily
- fix a limitation where `pl_tex_destroy` sometimes delayed destroying
  textures unnecessarily
- improve the performance of `pl_dispatch`'s code for assembling UBOs
- improve the performance of `pl_shader_av1_grain` by switching from
  SSBOs to texture LUTs, also improving compatibility with older GL
- improve the performance of `pl_shader_detect_peak` on GPUs with access
  to subgroups operations
- fix an issue where reinitializing shader state objects with different
  settings sometimes resulted in undefined behaviour
- fix some GLSL backwards compatibility issues
- slightly cut down on unnecessary image layout transitions
- add some miscellaneous debug print-outs, and improve the legibility of
  some existing log messages
- fix an issue where freeing buffer variables from shader descriptors
  could result in use-after-free
- add support for vulkan memory imports requiring dedication allocations
- slightly improve, and fix, the vulkan memory placement logic
- significantly improve the performance of `pl_tex_download` by
  importing the target host pointer directly
- improve the performance of small LUTs, especially for the non-compute
  polar fallback path
- prevent `pl_renderer` from unneccessarily applying a 3DLUT when both
  the input and output frames have the same ICC profile
- fix the behaviour of partially specified `pl_bit_encoding` structs
- fix the component ordering on some odd packed vulkan formats
- fix an issue where `pl_plane_data_from_mask` broke for high bit depths
- make `pl_renderer` consult the dither bit depth from the texture
  precision, if absent from `pl_bit_encoding`
- fix the behaviour of `pl_renderer` when sampling from textures with
  swapped component orders
- fix the implementation of PL_COLOR_SYSTEM_BT_2100_HLG
- fix some issues relating to missing includes
- enforce legality of image usage parameters on `pl_vulkan_wrap`
- fix an undesired shader double-compilation when using orthogonal
  scalers with subsampled chroma planes
- fix an issue where application of a 3DLUT cleared the alpha channel
- add a missing extension to `pl_vulkan_recommended_extensions`
- fix the plane alignment code for oddly sized subsampled chroma
- fix the poor precision of `pl_shader_dither` for high bit depths
- fix several possible overflows in the BT.2390 shader
- fix a bug where using a polar sampler to draw a scaled overlay onto a
  non-storable target neglected to disable compute shaders
- fix a bug where the renderer could sometimes alias when downscaling,
  in particular if both the upsampler and downsampler are set to bicubic
- improve performance of scalers by avoiding bilinear filtering
- reduce verbosity of memory allocations
- fix missing PL_FMT_CAP_BLENDABLE on opengl fbos
- significantly improve precision of float literals in shaders
- add better error checking to gl_pass_create