This is in preparation of the Framebuffer GL backend.
This is a just changing types and moving some code.
No logic is changed... almost... it just removes the context attach.
i.e: `gpu_context_add/remove_framebuffer()`
This is not needed for now and was even disabled in release.
This is part of T68990.
This replace `GPU_clear()` by `GPU_clear_color()` and `GPU_clear_depth()`.
Since we always set the clear value before clearing, it is unecessary
to track the clear color state.
Moreover, it makes it clearer what we clear the framebuffer to.
Issue introduced on fe045b2b77.
Since the stereoscopy compositing (anaglyph, ...) is only done for
viewports the VSE preview and compositor need to use viewports.
Reviewed by: dfelinto
Differential Revision: https://developer.blender.org/D8472