cssnode: Make widget node not copy the path

Instead, use gtk_widget_get_path() which makes GtkWidget cache the path.
This is a temporary solution until we can get rid of widget paths.

This increases memory usage quite noticably.

gtk/gtkcsswidgetnode.c

@@ -178,10 +178,8 @@ gtk_css_widget_node_init_matcher (GtkCssNode     *node,
   if (widget_node->widget == NULL)
     return FALSE;
 
-  *path_out = _gtk_widget_create_path (widget_node->widget);
-
   return _gtk_css_matcher_init (matcher,
-                                *path_out,
+                                gtk_widget_get_path (widget_node->widget),
                                 gtk_css_node_get_declaration (node));
 }