Blender Git Loki

OpenGL: bump gpu_extensions to GL 2.1, prepare for 3.x

Several changes. Tested working on Windows 10 GL 4.3 and MacOS 10.11 GL
2.1.

- document extensions used in this file
- some simple ARB/EXT suffix deletion
- stop checking for pre-2.1 features ? they?re available!
- convert old ARB shader API to the one adopted in GL 2.0
- remove checks for old (pre-R600) ATI cards
- choose GLSL version at runtime, between 1.2 and 1.5
- prefer GLSL 1.5 for geometry shaders, fall back to
EXT_geometry_shader4 if needed

Differential Revision: https://developer.blender.org/D1632