Exit remove_child_area when region is empty

When the region is empty we can return early, because there
is no more area to remove. This happens often for children
of scrolled windows (i.e. things that are clipped out.

gdk/gdkwindow.c

@@ -623,6 +623,11 @@ remove_child_area (GdkWindowObject *private,
       if (child == until)
 	break;
 
+      /* If region is empty already, no need to do
+	 anything potentially costly */
+      if (gdk_region_empty (region))
+	break;
+
       if (!GDK_WINDOW_IS_MAPPED (child) || child->input_only || child->composited)
 	continue;