Ball bounce strategy should be randomly picked

[wkeeling]

A custom bounce strategy can be supplied when a collidable sprite is added to the ball. If the ball collides with a single sprite with one of these bounce strategies, then this strategy is used to control how the ball bounces. However, if the ball collides with multiple sprites with each one having its own strategy, then the ball ignores the strategies and uses its default bounce calculation logic.

Rather than ignoring the bounce strategies, the ball should perhaps randomly select one of them to use for the bounce calculation.