/// <summary>
        /// Adds a new screen to the screen manager.
        /// </summary>
        public void AddScreen(GameScreen screen, PlayerIndex? controllingPlayer)
        {
            screen.ControllingPlayer = controllingPlayer;
            screen.ScreenManager = this;
            screen.IsExiting = false;

            // If we have a graphics device, tell the screen to load content.
            if (_isInitialized)
            {
                screen.LoadContent();
            }

            _screens.Add(screen);

            // update the TouchPanel to respond to gestures this screen is interested in
            TouchPanel.EnabledGestures = screen.EnabledGestures;

            screen.FirstRun = false;
        }
        /// <summary>
        /// Removes a screen from the screen manager. You should normally
        /// use GameScreen.ExitScreen instead of calling this directly, so
        /// the screen can gradually transition off rather than just being
        /// instantly removed.
        /// </summary>
        public void RemoveScreen(GameScreen screen)
        {
            // If we have a graphics device, tell the screen to unload content.
            if (_isInitialized)
            {
                screen.UnloadContent();
            }

            _screens.Remove(screen);
            _screensToUpdate.Remove(screen);

            // if there is a screen still in the manager, update TouchPanel
            // to respond to gestures that screen is interested in.
            if (_screens.Count > 0)
            {
                TouchPanel.EnabledGestures = _screens[_screens.Count - 1].EnabledGestures;
            }
        }