clutter-actor: Keep is_dirty unchanged for culled actors

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/1049
https://gitlab.gnome.org/GNOME/mutter/merge_requests/511

clutter/clutter/clutter-actor.c

@@ -4007,12 +4007,12 @@ clutter_actor_paint (ClutterActor *self)
                   pick_mode == CLUTTER_PICK_NONE))
     _clutter_actor_draw_paint_volume (self);
 
-done:
   /* If we make it here then the actor has run through a complete
      paint run including all the effects so it's no longer dirty */
   if (pick_mode == CLUTTER_PICK_NONE)
     priv->is_dirty = FALSE;
 
+done:
   if (clip_set)
     {
       CoglFramebuffer *fb = _clutter_stage_get_active_framebuffer (stage);