Added functions to the Game class to query the width and height of the main SDL_Surface*.
Fixed up the SinglePlayerGameState, it now has two paddles and can return to the MenuGameState.

 - The bottom paddle is controlled by the MousePaddleController. While the top paddle is static.
 - Pressing any key returns state to the MenuGameState.
 - Mouse is captured and hidden when the user starts a single player game.
 - Mouse is explicitly uncaptured and shown in the MenuGameState so it's consistent even when passed control from SinglePlayerGameState.