message: Make cursor unavailable on leave in `iced_winit`
  For now, we just set the cursor position to some negative coordinates.
  This is a temporary hack until we are able to encode cursor
  availability. Layers and/or multi-window support will make this
  apparent.
diff: @@ -188,9 +188,16 @@ where
  let mut messages = Vec::new();
  for event in events {
  - if let Event::Mouse(mouse::Event::CursorMoved { x, y }) = event {
  + match event {
  + Event::Mouse(mouse::Event::CursorMoved { x, y }) => {
  self.cursor_position = Point::new(x, y);
  }
  + Event::Mouse(mouse::Event::CursorLeft) => {
  + // TODO: Encode cursor availability
  + self.cursor_position = Point::new(-1.0, -1.0);
  + }
  + _ => {}
  + }
  self.root.widget.on_event(
  event,
  