More OpenGL refactors

Xaver Hugl requested to merge work/zamundaaa/gl2 into master

For details see the individual commits, but the rough goal of most commits in here is to simplify and reduce usage of GLPlatform, which gets in the way when multiple contexts are involved (like in the drm backend right now). I'll do more for that in future MRs.

Note that I haven't tested this on X11 yet because my X11 sessions (5.27 and dev) doesn't start for some reason

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