In a multi-monitor setup there is a separate paint run for each monitor.
If an actor doesn't intersect the first monitor painted then it is culled
out for that monitor to save time. Unfortunately this would mean
`clutter_actor_paint` was setting `is_dirty = FALSE` before the actor had
yet been painted on any monitor.
This meant that effects like `ClutterOffscreenEffect` were not receiving
the flag `CLUTTER_EFFECT_PAINT_ACTOR_DIRTY` when they should have, and
so would rightfully think they don't need to do a full internal
invalidation. So `ClutterOffscreenEffect`, and probably other effects,
did not repaint correctly unless on the first monitor in the list.
The fix is to simply avoid setting `is_dirty = FALSE` on those paint
runs where the actor has been culled out (`clutter_actor_continue_paint`
wasn't called). It is only safe to clear the flag after
`clutter_actor_continue_paint` has been called at least once per stage
paint.
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/1049https://gitlab.gnome.org/GNOME/mutter/merge_requests/511
ee507d9ab2