v1.21.0

This is a minor / house-keeping release, mostly to get some important
fixes into distros while also getting support for iGPUs requiring
non-coherent memory into a "stable" release.

Additions:
- `shaders.h` API users can now override the GLSL version
- add support for vulkan platforms with non-coherent memory (e.g. iGPUs)

Changes:
- default GLSL version is now 130 unless otherwise specified
- signature of `pl_shader_alloc` now uses a params struct
- PL_FIX_VER / PL_VERSION are no longer header constants
- the renderer will now automatically adjust deband grain scale for HDR

Fixes and performance improvements:
- on GLSL < 130, shaders now correctly use `texture2D` etc.
- fix a bug where async transfer was never actually used, due to
  incorrect alignment checks
- fixed some issues that could lead to symbol conflicts
- don't require large vertex buffers be host-visible (usually impossible)
- correctly detect swapchain image feature flags
- turned a few 1ms busy loops into infinite waits
- fix compilation on some platforms that require SPIRV-Tools
- fix vkCreateEvent error spam on MoltenVK
- bump queued command limit from 64 to 1024
This tag has no release notes.