Blender Git Loki

GPUTexture: GL Backend Isolation

This is a massive cleanup needed for vulkan support T68990. It provides:

- More meaningful enums with conversion functions.
- Less hacky supports of arrays and cubemaps (all considered layered).
- More inline with the stateless design of vulkan and modern GL.
- Methods Fallbacks are using framebuffer functions that are wrapped
instead of implementing inside the texture module.

What is not in there:
- API change.
- Samplers support (breaks a few effects).

# Conflicts:
# source/blender/gpu/GPU_texture.h