Stereoscopic viewport didn't support Color Manangement due recent
changes in the color management pipeline. In order to solve the issue we
will migrate the strereo rendering into the GPUViewport. This will share
some textures and reduce required GPU memory.
Reviewed By: fclem, dfelinto
Differential Revision: https://developer.blender.org/D6922
Put DefaultFramebufferList and DefaultTextureList into an own header
file. These structs were private, but also used by the window manager
and gpu package. The previous implementation we needed to keep these
structs in sync.
