clean up the callback in exit_grab() for macos

[xosxos]

Otherwise the variables moved into it will never be dropped, and for example, channels inside the callback will not receive.

[rustdesk]

Does this fix any bug in RustDesk?

[xosxos]

Well, I ran into the following issue:

let (tx, rx) = channel();
let tx = Arc::new(tx);

let callback = move |event: Event| -> Option<Event> {
    if something {
          tx.send(send_an_item);
          exit_grab();
    }
};

// Blocking
let _ = grab(callback);

drop(tx);

// Cannot receive items, because a reference to `tx` exists inside the boxed callback function
let receive_items = rx.iter().collect();

I suppose the question is: should the callback function live on the heap for the duration of the entire program regardless of grab_exit. As far as I could see, the callback cannot be called after the exit, and thus, it should not live. But I might be wrong as well!