Pinball domain can violate its statespace_limits

[amarack]

This should be easy to fix, but I just ran into it. In Pinball the ball dx/dy variable can leave the limits defined in statespace_limits due to collisions and the fact that in one location the value is not being clipped. Specifically, lines 451 and 452 in Pinball.py need to be wrapped in a call to self.ball._clip.