MetaWindowActor: unset the shaped texture overlay path for frameless windows

When a window loses its frame we must unset any overlay path previously set on
the shaped texture.

Not doing so would cause rendering glitches near the window corners in
e.g. chrome/chromium by changing the Appearance preference "Use system title
bar and borders" → "Hide system title bar and use compact borders".

https://bugzilla.gnome.org/show_bug.cgi?id=659477

src/compositor/meta-window-actor.c

@@ -2078,7 +2078,11 @@ update_corners (MetaWindowActor   *self,
   cairo_surface_t *surface;
 
   if (!priv->window->frame)
-    return;
+    {
+      meta_shaped_texture_set_overlay_path (META_SHAPED_TEXTURE (priv->actor),
+                                            NULL, NULL);
+      return;
+    }
 
   meta_window_get_outer_rect (priv->window, &outer);