separate opengl functions
as a preparation to replace glew. if https://www.graphics.rwth-aachen.de:9000/OpenFlipper-Free/OpenFlipper-Free/merge_requests/305 is merged, please also merge this one.
Edited by Martin Schultz
as a preparation to replace glew. if https://www.graphics.rwth-aachen.de:9000/OpenFlipper-Free/OpenFlipper-Free/merge_requests/305 is merged, please also merge this one.