Blender Git Loki

GLRefactor: Remove all GL types from gawains public headers.

In preparation for possible alternative back-ends, the dependency on glew.h needs to be confined to as few libraries as possible. This patch removes all gl types from the public gawain headers.

It'll need some more refactoring later on, but for now this will suffice.

Reviewer: brecht, fclem

Differential Revision: https://developer.blender.org/D3482