WIP: item: do basic occlusion culling for repaint scheduling

This is not necessarily the way we want to go, but it's the simplest to implement (because it requires no caching, no additional checking of Item lifetimes or anything like that), so it's at least worth testing. To test it, enable the fps effect and check how often and how much it repaints with for example a Firefox window playing a video while being covered by another (opaque) window. the fps effect isn't suitable for that, it only gets an already-culled repaint region from Scene.

WIP because efficiency should to be properly checked for. On my desktop I haven't noticed an increase in CPU usage, but that doesn't say a lot. Frame callbacks should also be adjusted, though that can be done separately.