Blender Git Loki

GL: Add fallback debug layer

This is to improve debugging on older hardware that may not support
4.3 debug capabilities (like Macs).

This avoids sprinkling glGetErrors manually. This might still be needed
to find the root cause since not all functions are covered.

This overrides the functions pointers that GLEW have already init time.

This is only enabled if using --debug-gpu option and the debug extension
are not available.