This is a major new release involving a significant overhaul of the
public API, full thread safety, support for Direct3D 11 / Win32 /
MSVC, native HDR output, and more.

The primary way of accessing most libplacebo objects has been revamped.
Rather than writing e.g. `const struct pl_tex *image`, users now simply
write `pl_tex image`. In addition, the `struct pl_context *ctx` was
universally refactored and replaced by `pl_log log`, with no concern
besides logging.

Direct3D 11 is now natively supported by libplacebo, and compiling on
Windows in general has been made more straightforward.

The entire API was revamped to be thread safe by default, even when
using OpenGL (assuming the user provides the appropriate context binding
callbacks). Accompanying this change, the `pl_queue` has been rewritten
with multi-threaded decoding loops in mind. See the accompanying
`plplay` demo for an illustration.

Finally, this release brings with it a lot of features for feature
parity with mpv, such as the ability to get pass stats / timings,
oversample scaling, and more.

- add a `void *user_data` field to `pl_frame`, with no further
  interpretation by libplacebo
- add `pl_queue_push_block`, useful for multi-threaded decoding loops
- add a `uint64_t timeout` to `pl_queue_update`, allowing it to
  block until frames are available even when not using a `get_frame`
- make `pl_queue_update` also return valid data when failing with
  PL_QUEUE_MORE, allowing access to partial/incomplete mix outputs
- allow calling `pl_render_image` on `image == NULL`, to render overlays
- add `pl_get_buffer2`, API-compatible with AVCodecContext.get_buffer2
- add support for blitting from/to several obscure formats such as
  a1rgb5 or 64-bit integer formats
- add `pl_tex_clear_ex` to support clearing integer textures
- add `pl_render_params.ignore_icc_profiles`, for debugging
- add `pl_queue_params.interpolation_threshold`, to allow disabling
  frame mixing when the source FPS approximately matches the display FPS
- always accept DRM_FORMAT_MOD_INVALID for texture imports
- add extra windowing functions to <libplacebo/filters.h>
- add the `pl_fmt.gatherable` capability
- add `pl_shader_sample_oversample`, a variant of nearest-neighbour
  that preserves pixel aspect ratios - good for pixel art
- add `pl_scale_filters` alongside `pl_frame_mixers`
- add support for compile-time specialization constants (`pl_constant`),
  and use them to speed up recompilation of shaders with hard-coded
- add `pl_render_params.dynamic_constants`, which lifts hard-coded
  constants to dynamic variables - useful for scenarios in which render
  parameters are expected to change very frequently
- add more PL_COLOR_TRC_GAMMAxx definitions
- implement full black point adaptation, even when not using ICC
  profiles, and infer this black-point-adapted BT.1886 instead of gamma
  2.2 as the default gamma curve for SDR files.
- add `pl_shader_res.description` and `pl_shader_res.steps`, containing
  more friendly names for shaders plus a detailed list of operations
  that shader is performing
- add callbacks to `pl_dispatch` and `pl_renderer` for informing users
  of executed passes and their execution times
- add support for the Direct3D 11 graphics API
- add `pl_swapchain_colorspace_hint`, replacing
  `pl_swapchain_hdr_metadata` as the new way to update swapchain
  colorspace metadata at runtime. This can be used to e.g. switch
  between HDR and SDR mode, for supported swapchains
- add `pl_peak_detect_params.minimum_peak`, allowing users to constrain
  the detected peak values to only be sensible (e.g. above 1.0)

- simplify the `shaderc` pkg-config check - rather than querying for
  `shaderc_shared.pc`, `shaderc_combined.pc` etc, simply check for
  `shaderc.pc`, matching upstream
- make almost everything thread-safe, and document the parts that
  aren't. In particular, almost all GPU state access is now thread safe,
  freeing up users to access `pl_gpu` instances from multiple threads,
  even when the underlying API is OpenGL
- deprecate `disable_overlay_sampling`, now effectively always true
- `struct pl_overlay` has been refactored completely to allow for
  overlays with more than one part per texture. The only way of using
  this struct is deprecated
- prefix `enum pl_queue_status` members by `PL_`, fixing an oversight in
  the previous version of this API
- make `pl_renderer` automatically clear the target image, freeing users
  from the responsibility of calling `pl_frame_clear` themselves. This
  can be controlled via three new fields in `pl_render_params`:
  `background_color`, `background_transparency` and `skip_target_clearing`
- rename `pl_context` to `pl_log`, and make its use optional. The old
  names have been deprecated. It now lives inside <libplacebo/log.h>
- add `typedefs` to all public-facing object types, o make them shorter
  to reference. For example, `const struct pl_tex *tex` is now simply
  `pl_tex tex`. This change affects almost every type of object in
  libplacebo. The old way of referencing these objects is still
  possible, but considered deprecated. Note that due to C++-specific
  reasons, C++ users *must* upgrade their codeto the new API style
- remove support for 64-bit float formats, which probably don't work on
  any Vulkan implementation
- stop contrast-limiting ICC profiles, instead assuming perceptual
  profiles have infinite contrast
- remove `pl_shader_signature` for being necessarily unsafe
- require Vulkan 1.1 as the minimum vulkan version
- rename `pl_oversample_frame_mixer` to `pl_filter_oversample`, and
  allow using it for image scaling as well
- change `pl_color_space_monitor` from PL_COLOR_TRC_GAMMA22 to UNKNOWN
- change the signature of `pl_shader_(de)linearize` slightly
- significantly increase the default strengths of the desaturation
  settings in`pl_color_map_params`, to mimic the hollywood feel
- refactor `pl_glsl_desc` and `pl_gpu_caps` completely, in favor of the
  new structs `pl_glsl_version` and `pl_gpu_limits`, with new members.
  The old API is still available for the time being, but deprecated

Fixes and performance improvements:
- fix possible use-after-free in `plplay`
- don't explode `pl_queue` on NaN/Infinity/weird PTS values
- fix edge case involving plane merging for cropped images
- improve `plplay` by using threaded libplacebo APIs
- several fixes for edge cases in <libplacebo/utils/libav.h> helpers
- several fixes for 32-bit platforms (e.g. integer overflows)
- fix blending edge cases involving overlays and alpha channels
- skip some unnecessary matrix multiplications for RGB content
- allow for some small backwards PTS jumps in `pl_queue_update`
- fix drawing overlays to subsampled targets
- `pl_dispatch` now garbage collects old, unused passes to free up RAM
- various improvements to debug/diagnonstic printouts
- allow blitting from e.g. 2D to 3D textures
- slightly improve small texture transfers in some emulated edge cases
- fix several possible hash collisions for generated shaders, making the
  shader dispatch mechanism significantly more robust
- properly allow building against vulkan headers without linking to the
  vulkan loader
- C++ compatibility for public headers
- improve performance of textureGather-based polar sampling, especially
  for radius 2 and 4
- fix vulkan texture handle capabilities check, again
- don't include superfluous pNext chains, fixes undefined behavior
- fix crash on edge case when shader compilation fails
- infer unsampled alpha channels as 1.0, rather than 0.0
- properly infer `target->color` in `pl_render_image`
- properly tag the OpenGL swapchain as pl_color_space_monitor
- fix possible crash in pl_get_detected_peak
- fix several edge cases in pl_render_image_mix relating to LUTs,
  3DLUTs, HDR peak detection and so on
- fix sig_peak inferral for HLG content
- several compatibility fixes for older GLSL, and GLES 2.0
- fix possible use-after-free in pl_shader_custom
- fix `pl_color_map_params.gamut_clipping` for HDR targets
- reduce the number of redundant color space transformations required
  for frame mixing
- fix edge case in pl_dispatch_save for some ancient GL drivers
- several fixes for LLVM/MinGW/MSVC
- fix bug where pl_pass_run incorrectly invalidated/accessed
  `params->target` even for compute shaders
- add windows compatibility to several demo programs
- fix the pl_fmt <-> DRM fourcc format mapping table
- fix inappropriate texture access function for generated samplers with
  the sampler2D API
- avoid generating spurious EGL errors when probing for EGL format
- fix various typis in the documentation