opengl: drop support for desktop OpenGL

Xaver Huglrequested to merge
work/zamundaaa/drop-desktop-gl into master

The various incompatibilities between desktop GL and OpenGL ES cause problems again and again. Code written and tested for one might not work for the other, ranging from things being rendered wrong to falling back to a software renderer or even functionality being completely broken.

Since we need to support OpenGL ES for some old hardware, going forward, KWin will only support OpenGL ES. This means everyone is running the same OpenGL code, and we can drop some practically redundant code as well.

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