wayland: destroy subsurfaces along with parents

Wayland subsurfaces can have other native window parents, but those need
to be destroyed along with the rest of the window hierarchy otherwise
an assert() is reached.

https://bugzilla.gnome.org/show_bug.cgi?id=774915

gdk/gdkwindow.c

@@ -1937,6 +1937,7 @@ _gdk_window_destroy_hierarchy (GdkWindow *window,
     case GDK_WINDOW_CHILD:
     case GDK_WINDOW_TEMP:
     case GDK_WINDOW_FOREIGN:
+    case GDK_WINDOW_SUBSURFACE:
       if (window->window_type == GDK_WINDOW_FOREIGN && !foreign_destroy)
 	{
 	  /* Logically, it probably makes more sense to send