This is in order to disolve GPU_draw.h into more meaningful code blocks.
All the Image related function are in `image_gpu.c`.
All the MovieClip related function are in `movieclip.c`.
The IMB module now has a connection with GPU. This is not strickly
necessary and the code could be move to `image_gpu.c` if needed.
The Image garbage collection is also ported to `image_gpu.c`.
This is also a bit of code cleanup, reorganisation.
Tried to be DRYed but avoid too much code change to (hopefully) minimize
breakage.
- GPU: remove TEXTARGET_CUBE_MAP, this is no longer used in the codebase.
- GPUTexture: Move compressed texture upload to gpu_texture.cc
- GPUTexture: Add per texture Anisotropic filtering switch
