v0.2.1

This is a bugfix release, intended to backport a number of easy and/or
critical fixes onto v0.2.0, since this is effectively the current 'LTS'
due to its use in VLC 3.0. Despite the included fixes, it's still not
recommended to make use of `pl_renderer` in this release. v0.4.0 is the
first release where that feature could be considered officially
supported.

For a full list of backported changes, see the git log. But aside from
cosmetics, comment fixes, here are the most noteworthy changes:

Fixes:
- Fix some typos in shaders that lead to miscompilation
- Work around the lack of mix(vecN, vecN, bvecN) on GLSL 120
- Avoid the use of platform-dependent locale-invariant printf() calls
  for shader generation. Instead, use a custom implementation
- Work around libshaderc locale dependence (currently only relevant when
  using vulkan)
- Fix various errors with anamorphic content handling
- Fix various errors when using vulkan
- Add some more sanitization to RA calls, and fix some of the existing
  ones
This tag has no release notes.