wayland: return from toplevel commit early if no new buffer

meta_wayland_surface_toplevel_commit has a lot of logic to handle
a new buffer getting attached as part of the commit.  None of
that code needs to run if there is no new buffer attached.

This commit short-circuits that case.

https://bugzilla.gnome.org/show_bug.cgi?id=761613

src/wayland/meta-wayland-surface.c

@@ -345,6 +345,12 @@ toplevel_surface_commit (MetaWaylandSurfaceRole  *surface_role,
 
   queue_surface_actor_frame_callbacks (surface, pending);
 
+  /* If there's no new buffer pending, then there's nothing else to
+   * do
+   */
+  if (!pending->newly_attached)
+    return;
+
   if (META_IS_WAYLAND_SURFACE_ROLE_WL_SHELL_SURFACE (surface->role))
     {
       /* For wl_shell, it's equivalent to an unmap. Semantics