window-actor: Add 'damaged' signal

Make it possible to listen for damage on a window actor. For X11, the
signal is emitted when damage is reported; for Wayland, it is emitted
when any of the surfaces associated with the window is damaged.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/752

src/compositor/meta-window-actor-private.h

@@ -93,4 +93,6 @@ void meta_window_actor_set_geometry_scale (MetaWindowActor *window_actor,
 
 int meta_window_actor_get_geometry_scale (MetaWindowActor *window_actor);
 
+void meta_window_actor_notify_damaged (MetaWindowActor *window_actor);
+
 #endif /* META_WINDOW_ACTOR_PRIVATE_H */

src/compositor/meta-window-actor.c

@@ -115,6 +115,7 @@ enum
 {
   FIRST_FRAME,
   EFFECTS_COMPLETED,
+  DAMAGED,
 
   LAST_SIGNAL
 };
@@ -219,6 +220,20 @@ meta_window_actor_class_init (MetaWindowActorClass *klass)
                   NULL, NULL, NULL,
                   G_TYPE_NONE, 0);
 
+  /**
+   * MetaWindowActor::damaged:
+   * @actor: the #MetaWindowActor instance
+   *
+   * Notify that one or more of the surfaces of the window have been damaged.
+   */
+  signals[DAMAGED] =
+    g_signal_new ("damaged",
+                  G_TYPE_FROM_CLASS (object_class),
+                  G_SIGNAL_RUN_LAST,
+                  0,
+                  NULL, NULL, NULL,
+                  G_TYPE_NONE, 0);
+
   pspec = g_param_spec_object ("meta-window",
                                "MetaWindow",
                                "The displayed MetaWindow",
@@ -1498,6 +1513,8 @@ meta_window_actor_process_x11_damage (MetaWindowActor    *self,
                                        event->area.y,
                                        event->area.width,
                                        event->area.height);
+
+  meta_window_actor_notify_damaged (self);
 }
 
 void
@@ -2111,3 +2128,9 @@ meta_window_actor_from_actor (ClutterActor *actor)
 
   return NULL;
 }
+
+void
+meta_window_actor_notify_damaged (MetaWindowActor *window_actor)
+{
+  g_signal_emit (window_actor, signals[DAMAGED], 0);
+}