Fix cursor to stay inside the game window.

I would really like to have the mouse cursor not be able to leave the game window. It happens even when I've been using small movements to play. I really could enjoy the game if I wasn't always unintentionally straying outside the window. That takes away from the enjoyment of the game. I was dead early on in stage 2 because of loss of control happening so much.
Thanks! I would really like the game except for that!