Port OpenGL backend

An OpenGL RA backend already exists for mpv's version of RA. But porting it is non-trivial:

1. Many RA API changes were made, most of which exist to make state more explicit. As such, more about e.g. OpenGL texture formats needs to be double-checked. This requires a lot of difficult cross-referencing between different OpenGL specs and versions.

2. A new public API (similar to libplacebo/vulkan.h) needs to be designed and implemented, in order to allow users to actually use it.