clutter: Keep actors dirty if a redraw was queued up during paint()

In the right combination of circumstances, and given 2 actors (parent
actor P with an offscreen effect and child actor C), we may have the
following situation happening:

- A redraw is queued on the actor C, actors C and P are marked as
  priv->is_dirty and priv->propagated_one_redraw.
- During paint() handling we paint actor P, priv->propagated_one_redraw
  is turned off.
- We recurse into child actor C, priv->propagated_one_redraw is turned
  off.
- A new redraw is queued on actor C, actors C and P are marked as
  priv->is_dirty and priv->propagated_one_redraw.
- The paint() method recurses back, actors C and P get priv->is_dirty
  disabled, priv->propagated_one_redraw remains set.
- At this point queueing up more redraws on actor C will not propagate
  up, because actor C has priv->propagated_one_redraw set, but the
  parent actor P has priv->is_dirty unset, so the offscreen effect will
  not get CLUTTER_EFFECT_PAINT_ACTOR_DIRTY and will avoid repainting
  actor C.

The end result is that actor C does not redraw again, despite requesting
redraws. This situation eventually resolves itself through e.g. relayouts
on actor P, but may take some time to happen.

In order to fix this, consider actors that did get a further redraw
request still dirty after paint().

Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2188
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2353>

clutter/clutter/clutter-actor.c

@@ -3840,8 +3840,10 @@ clutter_actor_paint (ClutterActor        *self,
   clutter_paint_node_paint (root_node, paint_context);
 
   /* 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 */
-  priv->is_dirty = FALSE;
+   * paint run including all the effects so it's no longer dirty,
+   * unless a new redraw was queued up.
+   */
+  priv->is_dirty = priv->propagated_one_redraw;
 }
 
 /**