[master-clock] Throttle if no redraw was performed

A flag in the master clock is now set whenever the dispatch caused an
actual redraw of a stage. If this flag is not set during the prepare
and check functions then it will resort to limiting the redraw
attempts to the default frame rate as if vblank syncing was
disabled. Otherwise if a timeline is running that does not cause the
scene to change then it would busy-wait with 100% CPU until the next
frame.

This fix was suggested by Owen Taylor in:

  http://bugzilla.openedhand.com/show_bug.cgi?id=1637

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>

clutter/clutter-master-clock.c

@@ -66,6 +66,8 @@ struct _ClutterMasterClock
    * a redraw on the stage and drive the animations
    */
   GSource *source;
+
+  guint updated_stages : 1;
 };
 
 struct _ClutterMasterClockClass
@@ -144,7 +146,8 @@ master_clock_next_frame_delay (ClutterMasterClock *master_clock)
   if (!master_clock_is_running (master_clock))
     return -1;
 
-  if (clutter_feature_available (CLUTTER_FEATURE_SYNC_TO_VBLANK))
+  if (clutter_feature_available (CLUTTER_FEATURE_SYNC_TO_VBLANK) &&
+      master_clock->updated_stages)
     {
       /* When we have sync-to-vblank, we count on that to throttle
        * our frame rate, and otherwise draw frames as fast as possible.
@@ -250,6 +253,8 @@ clutter_clock_dispatch (GSource     *source,
   stages = clutter_stage_manager_list_stages (stage_manager);
   g_slist_foreach (stages, (GFunc)g_object_ref, NULL);
 
+  master_clock->updated_stages = FALSE;
+
   /* Process queued events
    */
   for (l = stages; l != NULL; l = l->next)
@@ -262,7 +267,7 @@ clutter_clock_dispatch (GSource     *source,
    * is advanced.
    */
   for (l = stages; l != NULL; l = l->next)
-    _clutter_stage_do_update (l->data);
+    master_clock->updated_stages |= _clutter_stage_do_update (l->data);
 
   g_slist_foreach (stages, (GFunc)g_object_unref, NULL);
   g_slist_free (stages);
@@ -298,6 +303,8 @@ clutter_master_clock_init (ClutterMasterClock *self)
   source = clutter_clock_source_new (self);
   self->source = source;
 
+  self->updated_stages = TRUE;
+
   g_source_set_priority (source, CLUTTER_PRIORITY_REDRAW);
   g_source_set_can_recurse (source, FALSE);
   g_source_attach (source, NULL);

clutter/clutter-private.h

@@ -174,7 +174,7 @@ ClutterStageWindow *_clutter_stage_get_default_window   (void);
 void                _clutter_stage_maybe_setup_viewport (ClutterStage       *stage);
 void                _clutter_stage_maybe_relayout       (ClutterActor       *stage);
 gboolean            _clutter_stage_needs_update         (ClutterStage       *stage);
-void                _clutter_stage_do_update            (ClutterStage       *stage);
+gboolean            _clutter_stage_do_update            (ClutterStage       *stage);
 
 void     _clutter_stage_queue_event           (ClutterStage *stage,
 					       ClutterEvent *event);

clutter/clutter-stage.c

@@ -486,7 +486,7 @@ _clutter_stage_process_queued_events (ClutterStage *stage)
  *
  * Determines if _clutter_stage_do_update() needs to be called.
  *
- * Return value: %TRUE if the stages need layout or painting
+ * Return value: %TRUE if the stage need layout or painting
  */
 gboolean
 _clutter_stage_needs_update (ClutterStage *stage)
@@ -505,18 +505,20 @@ _clutter_stage_needs_update (ClutterStage *stage)
  * @stage: A #ClutterStage
  *
  * Handles per-frame layout and repaint for the stage.
+ *
+ * Return value: %TRUE if the stage was updated
  */
-void
+gboolean
 _clutter_stage_do_update (ClutterStage *stage)
 {
   ClutterStagePrivate *priv;
 
-  g_return_if_fail (CLUTTER_IS_STAGE (stage));
+  g_return_val_if_fail (CLUTTER_IS_STAGE (stage), FALSE);
 
   priv = stage->priv;
 
   if (!priv->redraw_pending)
-    return;
+    return FALSE;
 
   /* clutter_do_redraw() will also call maybe_relayout(), but since a relayout
    * can queue a redraw, we want to do the relayout before we clear the
@@ -539,6 +541,8 @@ _clutter_stage_do_update (ClutterStage *stage)
 
       CLUTTER_CONTEXT ()->redraw_count = 0;
     }
+
+  return TRUE;
 }
 
 static void