[MP] Rend2: Removed unneccessary RB_SetGL2D extern
[MP] Rend2: Removed a load of extension checks

This commit removes checking for extensions which are now part of OpenGL
3.2, in preparation for function renaming (removing ARB/EXT suffixes).
[MP] Rend2: Rename extension funcs to core ones

This commit renames all extension functions which are part of OpenGL 3.2
with their core names. It also adds a lot of new API functions which are
available in 3.2.