Blender Git Loki

Initial introduction of DRWLayer abstraction

Used to wrap different drawing steps, i.e. the main scene drawing,
editor overlays and debug text. Later overlays should get their own
layer.
Right now only the drawing commands are wrapped into per layer
callbacks. The idea is to let each draw into different framebuffers and
to composit them together at the end, so that we can avoid redrawing all
layers if only one needs updating.