This is a necessary step for EEVEE's new arch. This moves more data
to the draw manager. This makes it easier to have the render or draw
engines manage their own data.
This makes more sense and cleans-up what the GPUViewport holds
Also rewrites the Texture pool manager to be in C++.
This also move the DefaultFramebuffer/TextureList and the engine related
data to a new `DRWViewData` struct. This struct manages the per view
(as in stereo view) engine data.
There is a bit of cleanup in the way the draw manager is setup.
We now use a temporary DRWData instead of creating a dummy viewport.
Development: fclem, jbakker
Differential Revision: https://developer.blender.org/D11966
