Draft: Allow direct scanout for non-fullscreen clients using overlay planes
So we can skip compositing in common cases such as maximized clients.
Right now this is limited to opaque clients - and only happens as long as there is no stage damage outside the choosen surface.
Closes #61
This is early WIP but already works reasonable in non-rotated single-monitor setups. Just putting this MR already out for people who would like to play with it.