billiard: Cancel velocity downward when shooting to reduce jumping

We couldn't totally eliminate the chance to cause jump shot even if
the cue hits exact center of gravity of the ball.  Probably the physics
simulation omits something that shouldn't be omitted.
billiard: Balls can get potted when entering pocket holes

Potted balls do not interact with other balls anymore.
billiard: Reset the cue ball if it's potted