Blender Git Loki

Draw-layers: Get it all to work!

Create the framebuffers for caching viewport rendering results, and
"composite" all layers together once each is drawn. Compositing is a bit
of a big word for this, since we basically just draw all layers on top
of each other with alpha blending.
After talking with Jeroen, we decided to keep things simple for now, we
can add more features (like per-layer color-management) and do small
improvements later.