widget: Add default snapshot implementation

gtk_widget_snapshot will snapshot the css box, the default snapshot
vfunc now gtk_widget_snapshot_child's all child widgets.

gtk/gtkwidget.c

@@ -940,7 +940,12 @@ static void
 gtk_widget_real_snapshot (GtkWidget   *widget,
                           GtkSnapshot *snapshot)
 {
-  /* nothing to do here */
+  GtkWidget *child;
+
+  for (child = _gtk_widget_get_first_child (widget);
+       child != NULL;
+       child = _gtk_widget_get_next_sibling (child))
+    gtk_widget_snapshot_child (widget, child, snapshot);
 }
 
 static void