wayland: Send frame-callbacks to hidden surfaces when screen-casted

Screen casted hidden surfaces currently don't receive frame-callbacks as they are, unlike ClutterClones, not considered by clutter_actor_is_effectively_on_stage_view().

Thus special-case them. This is a minimal but hopefully solid solution for backward compatibility. It's not optimal in multi-monitor setups yet, as clients will receive frame-callbacks for every stage view, making them draw at higher rate than necessary. However, that's arguably usually already the case: optimally we'd only want them to redraw at the stream framerate, not synced to any stage view. So this is probably acceptable as an interim solution.

Closes #2538 (closed)