This release backports a number of bug fixes from master affecting the
v2.72.0 release, most notably extending the range of supported glslang
versions, as well as fixing the 3DLUT/ICC generation code.

Bug fixes:
- compatibility with glslang >= 8.13.3743
- compatibility with new glslang semantic versioning scheme
- fix broken shaders on some versions of GLES by defaulting to 32-bit
  precision for floating point math
- fix the vulkan API version passed to shaderc
- fix the extension check for glInvalidateTexImage
- fix pl_tex_create on older GLES versions
- fix OpenGL logging thread safety
- fix undefined memcmp() in pl_shader_av1_grain
- fix pl_render_target.repr being ignored by pl_render_image
- fix 3DLUT generation code (generated corrupt LUTs in all cases)
- fix symbol visibility on some versions of GCC
- fix potential overflow in BT.2390 shader
- properly restrict pl_shader_sample_polar to GLSL >= 130
- fix vulkan function loading of promoted core functions

Other changes:
- tiny performance gain in 3DLUT generation
- pl_opengl_create now logs GL_EXTENSIONS
- log some additional VkResult enum members
- improve several vulkan log messages by using friendly names of enums
- add a copy of the config.h variables to the libplacebo.pc file
This tag has no release notes.