cssnode: Clear widget path more aggressively

When recomputing CSS, we need a correct widget path in the fallback mode
where we're still using widget paths.
So we need to invalidate it everytime it actually changes, and not just
when emitting the style-updated signal.

Fixes css-match-regions reftest.

gtk/gtkcsswidgetnode.c

@@ -54,6 +54,9 @@ gtk_css_widget_node_style_changed (GtkCssNode   *cssnode,
 
   node = GTK_CSS_WIDGET_NODE (cssnode);
 
+  if (node->widget)
+    gtk_widget_clear_path (node->widget);
+
   GTK_CSS_NODE_CLASS (gtk_css_widget_node_parent_class)->style_changed (cssnode, old_style, new_style);
 
   diff = gtk_css_style_get_difference (new_style, old_style);