cssnode: Automatically recreate style on get_style() call

When the style is invalid, redo it.

Make this a vfunc, so the widget nodes can opt out.

gtk/gtkcsspathnode.c

@@ -24,13 +24,10 @@
 G_DEFINE_TYPE (GtkCssPathNode, gtk_css_path_node, GTK_TYPE_CSS_NODE)
 
 static void
-gtk_css_path_node_invalidate (GtkCssNode   *node,
-                              GtkCssChange  change)
+gtk_css_path_node_invalidate (GtkCssNode *node)
 {
   GtkCssPathNode *path_node = GTK_CSS_PATH_NODE (node);
 
-  gtk_css_node_set_style (node, NULL);
-
   if (path_node->context)
     {
       G_GNUC_BEGIN_IGNORE_DEPRECATIONS;