RenderLoop: restrict repaint scheduling with fullscreen windows

With an opaque fullscreen window we can be sure that items under it don't actually require us to repaint. This should yield some small efficiency improvements and resolves stutter with adaptive sync.

This is pretty much just a cheap hack around the fact that we don't have a proper scene graph yet. However, it's also easily backportable

BUG: 443872 FIXED-IN: 5.23.3