scene optimizations

Supersedes: !3488 (closed)
Closes: #3144 (closed)
Based on: !3509 (merged)

The set of optimizations implemented by this series:

• Only parts of the scene node that are visible will only get damaged.
• While rendering, only the visible parts of the nodes will actually be rendered. Everything else will be culled.
• output enter/leave events are based on a node's visibility.
• frame done events are only sent for visible nodes.
• Direct scan out will ignore invisible nodes above a proper candidate and all stuff below it if that candidate is opaque or its background is explicitly black through a rect for example.