Some Wayland frame callback fixes

This merge request fixes two issues: weston toytoolkit clients fail to maximize and off-screen damage may get window content stuck.

It works by moving the emitting of frame callbacks away from actors to the frame clock "update" phase more or less, meaning it doesn't depend on any damage or we actually painting the stage. Instead we use the meta_surface_actor_is_obscured() check to decide whether a frame callback should be emitted or postponed.