Provide Renderer to Widget::on_event

This allows us to implement configurable event processing that adapts to
different rendering strategies.

native/src/widget.rs

@@ -117,6 +117,7 @@ where
         _layout: Layout<'_>,
         _cursor_position: Point,
         _messages: &mut Vec<Message>,
+        _renderer: &Renderer,
     ) {
     }
 }