wayland: Keep last scale factor on surfaces after it left all outputs

This can be triggered on workspace switches, and on hidpi results in
the scale factor being reset to 1 while the window is not in the
current workspace.

https://bugzilla.gnome.org/show_bug.cgi?id=774476

gdk/wayland/gdkwindow-wayland.c

@@ -1235,7 +1235,8 @@ surface_leave (void              *data,
 
   impl->display_server.outputs = g_slist_remove (impl->display_server.outputs, output);
 
-  window_update_scale (window);
+  if (impl->display_server.outputs)
+    window_update_scale (window);
 }
 
 static const struct wl_surface_listener surface_listener = {