message: Support null renderer
diff: @@ -61,11 +61,21 @@ impl text::Renderer for Null {
  }
  impl scrollable::Renderer for Null {
  - fn scrollbar_grab(
  + fn scrollbar_bounds(
  &self,
  _bounds: Rectangle,
  _content_bounds: Rectangle,
  _offset: u32,
  + ) -> (Rectangle, Rectangle) {
  + Default::default()
  + }
  +
  + fn scrollbar_grab(
  + &self,
  + _bounds: Rectangle,
  + _content_bounds: Rectangle,
  + _background_bounds: Rectangle,
  + _scroller_bounds: Rectangle,
  _cursor_position: Point,
  ) -> Option<ScrollbarGrab> {
  None
  