Fixes render of debug scene.

(Need to debug why the debug scene needs to be drawn in a separate call
to RenderDisplayList.  Not sure why it can't be loaded onto the end of
the main one)
"Blit" as a render pipeline stage.

Still arguing with myself over whether an explicit "blit from scene to
window" should be required in a pipeline.  Or whether, in fact, the
whole hardcoded 'scene' and 'window' even make sense, and the 'scene'
should be pulled out of the renderer entirely.