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.

added 4 commits

• 0a0b7e73...7cd7dcb5 - 2 commits from branch master
• a54cfc6a - Merge branch 'master' into featureSeparateGlew
• 50a9bb66 - reverted changed include order.

Compare with previous version

mentioned in merge request OpenFlipper-Free!305 (merged)

changed the description

assigned to @moebius

mentioned in commit 2ef6a7d5

merged