csspathnode: Handle context going away

Sometimes path nodes can survive longer than the style context that
created them. Don't crash in those cases.

Fixes startup of mutter.

Testcase included.

https://bugzilla.gnome.org/show_bug.cgi?id=746407

gtk/gtkcsspathnode.c

@@ -156,6 +156,17 @@ gtk_css_path_node_new (GtkStyleContext *context)
   return GTK_CSS_NODE (node);
 }
 
+void
+gtk_css_path_node_unset_context (GtkCssPathNode *node)
+{
+  gtk_internal_return_if_fail (GTK_IS_CSS_PATH_NODE (node));
+  gtk_internal_return_if_fail (node->context != NULL);
+
+  node->context = NULL;
+
+  gtk_css_node_invalidate_style_provider (GTK_CSS_NODE (node));
+}
+
 void
 gtk_css_path_node_set_widget_path (GtkCssPathNode *node,
                                    GtkWidgetPath  *path)