When the style is invalid, redo it. Make this a vfunc, so the widget nodes can opt out.
Remove it from GtkStyleContext.
Handle invalidation of node inside the CssNode code, don't do it in the stylecontext.
- GtkCssWidgetNode for style contexts owned by a widget - GtkCssPathNode for style contexts using a GtkWidgetPath - GtkCssTransientNode for nodes created with gtk_style_context_save()/restore()
