libkwineffects: lots of OpenGL refactoring

Xaver Hugl requested to merge work/zamundaaa/kwinglutils-refactors into master

This MR

  • splits kwinglutils into multiple files
  • removes functionality meant for legacy OpenGL from GLVertexBuffer
  • removes streaming buffer etc from GLVertexBuffer in favor of just calling glBufferData. This greatly simplifies the code and most likely has 0 impact on performance
  • simplifies the index buffer implementation by always generating the index array at runtime and also always using glBufferData
  • enables error message printing even if not requested with the env var. While it might make sense to suppress debug messages without it, we always want to know when there's errors

For the details look at the commits themselves.

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