clutter/stage: Avoid queueing stage updates when they're already queued

We're using clutter_stage_schedule_update() now from ClutterActor to kick
off the stage updating machinery when a redraw needs to happen.

This introduced a bunch of unnecessary calls to
clutter_stage_schedule_update() and thus
clutter_stage_view_schedule_update() when multiple actors request redraws
during the same stage update cycle, which is a very common case.

Cut off all those unnecessary calls by bailing out in
clutter_stage_schedule_update() when updates are already queued.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2679>

clutter/clutter/clutter-stage-private.h

@@ -60,7 +60,7 @@ void                clutter_stage_emit_before_paint      (ClutterStage
 void                clutter_stage_emit_after_paint       (ClutterStage          *stage,
                                                           ClutterStageView      *view,
                                                           ClutterFrame          *frame);
-void                clutter_stage_emit_after_update      (ClutterStage          *stage,
+void                clutter_stage_after_update           (ClutterStage          *stage,
                                                           ClutterStageView      *view,
                                                           ClutterFrame          *frame);
 

clutter/clutter/clutter-stage-view.c

@@ -1280,7 +1280,7 @@ handle_frame_clock_frame (ClutterFrameClock *frame_clock,
   priv->needs_update_devices = FALSE;
 
   _clutter_run_repaint_functions (CLUTTER_REPAINT_FLAGS_POST_PAINT);
-  clutter_stage_emit_after_update (stage, view, frame);
+  clutter_stage_after_update (stage, view, frame);
 
   return clutter_frame_get_result (frame);
 }

clutter/clutter/clutter-stage.c

@@ -137,6 +137,8 @@ struct _ClutterStagePrivate
 
   int update_freeze_count;
 
+  gboolean update_scheduled;
+
   GHashTable *pointer_devices;
   GHashTable *touch_sequences;
 
@@ -518,11 +520,15 @@ clutter_stage_emit_after_paint (ClutterStage     *stage,
 }
 
 void
-clutter_stage_emit_after_update (ClutterStage     *stage,
+clutter_stage_after_update (ClutterStage     *stage,
                             ClutterStageView *view,
                             ClutterFrame     *frame)
 {
+  ClutterStagePrivate *priv = stage->priv;
+
   g_signal_emit (stage, stage_signals[AFTER_UPDATE], 0, view, frame);
+
+  priv->update_scheduled = FALSE;
 }
 
 static gboolean
@@ -633,18 +639,14 @@ _clutter_stage_queue_event (ClutterStage *stage,
                             gboolean      copy_event)
 {
   ClutterStagePrivate *priv;
-  gboolean first_event;
 
   g_return_if_fail (CLUTTER_IS_STAGE (stage));
 
   priv = stage->priv;
 
-  first_event = priv->event_queue->length == 0;
-
   g_queue_push_tail (priv->event_queue,
                      copy_event ? clutter_event_copy (event) : event);
 
-  if (first_event)
   clutter_stage_schedule_update (stage);
 }
 
@@ -778,7 +780,6 @@ clutter_stage_queue_actor_relayout (ClutterStage *stage,
 {
   ClutterStagePrivate *priv = stage->priv;
 
-  if (priv->pending_relayouts == NULL)
   clutter_stage_schedule_update (stage);
 
   priv->pending_relayouts = g_slist_prepend (priv->pending_relayouts,
@@ -2452,12 +2453,19 @@ _clutter_stage_get_default_window (void)
 void
 clutter_stage_schedule_update (ClutterStage *stage)
 {
+  ClutterStagePrivate *priv = stage->priv;
   ClutterStageWindow *stage_window;
+  gboolean first_event;
   GList *l;
 
   if (CLUTTER_ACTOR_IN_DESTRUCTION (stage))
     return;
 
+  first_event = priv->event_queue->length == 0;
+
+  if (priv->update_scheduled && !first_event)
+    return;
+
   stage_window = _clutter_stage_get_window (stage);
   if (stage_window == NULL)
     return;
@@ -2468,6 +2476,8 @@ clutter_stage_schedule_update (ClutterStage *stage)
 
       clutter_stage_view_schedule_update (view);
     }
+
+  priv->update_scheduled = TRUE;
 }
 
 ClutterPaintVolume *