widget: Cache the render node

widget: Cache the render node

This requires a bunch of refactorings:

1. Don't pass the current clip region to gtk_widget_snapshot()
   so we don't create full widget contents
3. Have a widget->priv->draw_needed that we invalidate on every
   queue_draw() call and set on every snapshot()
2. In queue_draw(), walk the widget chain to invalidate the
   render nodes of all parents