window-actor: Make meta_window_actor_get_image fastpath reachable

MetaWindowActor previously peeked at the number of child Actors to
determine the number of surfaces. The following commit rearranged the
tree such that MetaWindowActorWayland always has two Actors. This change
lets the subclass determine if the main surface describes the whole
window.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2338>

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

@@ -33,6 +33,8 @@ struct _MetaWindowActorClass
                       gboolean         frozen);
   void (*update_regions) (MetaWindowActor *actor);
   gboolean (*can_freeze_commits) (MetaWindowActor *actor);
+
+  gboolean (*is_single_surface_actor) (MetaWindowActor *actor);
 };
 
 typedef enum

src/compositor/meta-window-actor-wayland.c

@@ -207,6 +207,12 @@ meta_window_actor_wayland_can_freeze_commits (MetaWindowActor *actor)
   return FALSE;
 }
 
+static gboolean
+meta_window_actor_wayland_is_single_surface_actor (MetaWindowActor *actor)
+{
+  return clutter_actor_get_n_children (CLUTTER_ACTOR (actor)) == 1;
+}
+
 static void
 meta_window_actor_wayland_dispose (GObject *object)
 {
@@ -245,6 +251,7 @@ meta_window_actor_wayland_class_init (MetaWindowActorWaylandClass *klass)
   window_actor_class->set_frozen = meta_window_actor_wayland_set_frozen;
   window_actor_class->update_regions = meta_window_actor_wayland_update_regions;
   window_actor_class->can_freeze_commits = meta_window_actor_wayland_can_freeze_commits;
+  window_actor_class->is_single_surface_actor = meta_window_actor_wayland_is_single_surface_actor;
 
   object_class->dispose = meta_window_actor_wayland_dispose;
 }

src/compositor/meta-window-actor-x11.c

@@ -1534,6 +1534,12 @@ meta_window_actor_x11_can_freeze_commits (MetaWindowActor *actor)
   return clutter_actor_is_mapped (clutter_actor);
 }
 
+static gboolean
+meta_window_actor_x11_is_single_surface_actor (MetaWindowActor *actor)
+{
+  return clutter_actor_get_n_children (CLUTTER_ACTOR (actor)) == 1;
+}
+
 static void
 meta_window_actor_x11_set_property (GObject      *object,
                                     guint         prop_id,
@@ -1710,6 +1716,7 @@ meta_window_actor_x11_class_init (MetaWindowActorX11Class *klass)
   window_actor_class->set_frozen = meta_window_actor_x11_set_frozen;
   window_actor_class->update_regions = meta_window_actor_x11_update_regions;
   window_actor_class->can_freeze_commits = meta_window_actor_x11_can_freeze_commits;
+  window_actor_class->is_single_surface_actor = meta_window_actor_x11_is_single_surface_actor;
 
   actor_class->paint = meta_window_actor_x11_paint;
   actor_class->get_paint_volume = meta_window_actor_x11_get_paint_volume;

src/compositor/meta-window-actor.c

@@ -1508,6 +1508,12 @@ create_framebuffer_from_window_actor (MetaWindowActor  *self,
   return framebuffer;
 }
 
+static gboolean
+meta_window_actor_is_single_surface_actor (MetaWindowActor *self)
+{
+  return META_WINDOW_ACTOR_GET_CLASS (self)->is_single_surface_actor (self);
+}
+
 /**
  * meta_window_actor_get_image:
  * @self: A #MetaWindowActor
@@ -1541,7 +1547,7 @@ meta_window_actor_get_image (MetaWindowActor *self,
 
   stex = meta_surface_actor_get_texture (priv->surface);
   if (!meta_shaped_texture_should_get_via_offscreen (stex) &&
-      clutter_actor_get_n_children (actor) == 1)
+      meta_window_actor_is_single_surface_actor (self))
     {
       MetaRectangle *surface_clip = NULL;