st/viewport: Invalidate the cached paint volume of ClutterActor

Since StViewport uses the value of the StAdjustment to create its custom
paint volume, ClutterActors newly introduced proper caching of paint
volumes doesn't get notifed about changes to that paint volume and will
simply reuse the cached old one.

So make use of the newly introduced
clutter_actor_invalidate_paint_volume() method to make sure ClutterActor
will ask for the paint volume again on the next paint.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1484>

src/st/st-viewport.c

@@ -81,6 +81,7 @@ adjustment_value_notify_cb (StAdjustment *adjustment,
                             StViewport   *viewport)
 {
   clutter_actor_invalidate_transform (CLUTTER_ACTOR (viewport));
+  clutter_actor_invalidate_paint_volume (CLUTTER_ACTOR (viewport));
   clutter_actor_queue_relayout (CLUTTER_ACTOR (viewport));
 }