wayland/keyboard: Accept key down serial after key up for popups

If a client maps a popup in response to a key-down event, but the mapping doesn't occur until after the user has already released the same button, we'd immediately dismiss the popup. This is problematic, as one often presses and releases a key quite quickly, meaning any popup mapped on key-down are likely to be dismissed.

Avoid this race condition by accepting serials for key down events, if the most recent key-up event had the same keycode.