clutter/paint-volume: Don't over-enlarge 2D actorless paint volumes

A 2D actorless paint volume can't ever need enlarge_for_effects because it has no depth. Clamping to the pixel boundary is sufficient in this case and avoids extending volumes on the edge of the view into the next view. Which then avoids unnecessary secondary monitor updates.

Paint volumes correctly become actorless where clutter_actor_finish_layout calls _clutter_paint_volume_transform_relative.

Relates to: gnome-shell#6819 (closed) and fixes it when combined with gnome-shell!2828 (merged)