cssnode: Add hackery to allow bypassing widget paths

If we know the parent's get_path_for_child() implementation is safe to
be used with GtkCssNode because it doesn't do anything special, we do
that. Unfortunately that requires whitelisting vfuncs because the vfunc
is public API so anyone can override it.

gtk/gtkcsswidgetnode.c

@@ -168,6 +168,38 @@ gtk_css_widget_node_validate (GtkCssNode       *node,
   return changes;
 }
 
+typedef GtkWidgetPath * (* GetPathForChildFunc) (GtkContainer *, GtkWidget *);
+
+static gboolean
+widget_needs_widget_path (GtkWidget *widget)
+{
+  static GetPathForChildFunc funcs[1];
+  GtkWidget *parent;
+  GetPathForChildFunc parent_func;
+  guint i;
+
+  if (G_UNLIKELY (funcs[0] == NULL))
+    {
+      i = 0;
+      funcs[i++] = GTK_CONTAINER_CLASS (g_type_class_ref (GTK_TYPE_CONTAINER))->get_path_for_child;
+
+      g_assert (i == G_N_ELEMENTS (funcs));
+    }
+
+  parent = gtk_widget_get_parent (widget);
+  if (parent == NULL)
+    return FALSE;
+
+  parent_func = GTK_CONTAINER_GET_CLASS (GTK_CONTAINER (parent))->get_path_for_child;
+  for (i = 0; i < G_N_ELEMENTS (funcs); i++)
+    {
+      if (funcs[i] == parent_func)
+        return FALSE;
+    }
+
+  return TRUE;
+}
+
 gboolean
 gtk_css_widget_node_init_matcher (GtkCssNode     *node,
                                   GtkCssMatcher  *matcher)
@@ -177,6 +209,9 @@ gtk_css_widget_node_init_matcher (GtkCssNode     *node,
   if (widget_node->widget == NULL)
     return FALSE;
 
+  if (!widget_needs_widget_path (widget_node->widget))
+    return GTK_CSS_NODE_CLASS (gtk_css_widget_node_parent_class)->init_matcher (node, matcher);
+
   return _gtk_css_matcher_init (matcher,
                                 gtk_widget_get_path (widget_node->widget),
                                 gtk_css_node_get_declaration (node));