opengl: implement exporting and importing through EGL
Implements PL_HANDLE_DMA_BUF for opengl.
This commit also introduces a public-facing concept of DRM fourccs,
which are useful/needed by OpenGL to mediate the texture format. We
blend this into the pl_fmt concept because of parallels to how this is
done in other APIs, and because pl_fmt is already required to specify
upon import.
See !116 (closed) for discussion.
Co-authored-by: Niklas Haas git@haasn.xyz
This replaces !116 (closed)