This way it is way clearer what each viewport state is. There is
no more save and reset. The scissor test is also saved per
framebuffer.
The only rule to remember is that the viewport state (size and
origin) is reset for both the viewport and scissor when a texture
is attached or detached from an attachment slot.
This make use of the GLStateStack functions for:
- `GPU_blend()`
- `GPU_blend_set_func()`
- `GPU_blend_set_func_separate()`
The goal is to unify them using an explicit state setting.
This will remove the need to use obscure blend functions
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`.
