This is needed to establish explicit connection between an item and the
scene it belongs to. For now, the scene must be known at the item
construction time. Perhaps it can be improved in the future by items
inheriting their scene from the parent item, but the scene would need to
be refactored more so there's a root item or something like that.
Currently, Item schedules repaints taking output geometry into account,
but that's going to work poorly for cases other than workspace scene.
Moving from Output to SceneDelegate also allows us to unify some X11 and
Wayland specific code paths in Item.
Furthermore, it ensures that effects->paintScreen() won't remove pending
repaints for the output.
