clutter-actor: Remember and propagate is_dirty when mapped

When an actor was unmapped it would lose track of whether its contents were dirty because we return early from _clutter_actor_queue_redraw_full without setting is_dirty. And even if that actor was not offscreened, being unmapped it would fail to propagate is_dirty to its ancestors who might also be unmapped and offscreened. This resulted in (for example) the GNOME panel not updating after returning from fullscreen.

Now we remember the is_dirty flag for unmapped actors and propagate it up the hierarchy as soon as they become mapped.

Closes: gnome-shell#2973 (closed)