core: Update frame style when _GTK_THEME_VARIANT changes

When the _GTK_THEME_VARIANT property changes, rather than just
updating the window's theme_variant property, update its frame
style as well, so that the window decoration reflects the requested
variant. As the initial properties of a window may be read before
its frame is created, there will be cases where the change is not
picked up initially.

https://bugzilla.gnome.org/show_bug.cgi?id=645355

src/core/window-props.c

@@ -1540,6 +1540,9 @@ reload_gtk_theme_variant (MetaWindow    *window,
         window->gtk_theme_variant = g_strdup (requested_variant);
       else
         window->gtk_theme_variant = NULL;
+
+      if (window->frame)
+        meta_ui_update_frame_style (window->screen->ui, window->frame->xwindow);
     }
 }