v2.43.1

This release backports a number of bug fixes from master affecting the
v2.43.0 release, and also slightly extends the range of backwards
compatibility with respect to vulkan-headers versions. (This change is
not on master, so this can be seen as a compatibility release)

Bug fixes:
- fix GLSL version checks when overriding GLSL versions with
  `pl_shader_params.glsl`
- fix minor order-of-operations issue in AV1 grain synthesis
- fix glInvalidate* backwards compatibility issue
- fix compilation with older vulkan-headers (e.g. ubuntu 18.04)
- fix a bug where pl_shader_encode_color sometimes uploaded and applied
  an identity matrix unnecessarily
- work around upstream issues with MSL shader compilation by ensuring
  all buffer elements are sorted by offset
- fix hypothetical edge case with swapchain image usages forbidden by
  the swapchain image format
- fix an issue where blending overlays caused the rest of the image to
  be discarded

Other changes:
- log the vulkan surface selection choices
- log the overall library version on initialization
- add a few more pl_gpu validation checks
- significantly reduce the CPU consumpation of blocking texture downloads
This tag has no release notes.