Scrolling the app grid damages adjacent monitors

Scrolling the app grid damages adjacent monitors which otherwise don't need repainting. You can see this with CLUTTER_PAINT=damage-region, although I first noticed it as reduced performance when adding a second monitor.

This seems to have something to do with the allocation size of the app grid being bigger (by design) than the viewport it is in. Since the viewport clipping only happens at paint time, spurious damage gets accumulated on stage views that actually don't contain the viewport.