Add cheese-ball.
Use general alpha-test functionality for ball tests

This removes the ball-specific alpha-test system in favor of the newer
material-based capabilities, thereby also eliminating the possibility
of conflicts between the two systems.
Merge pull request #75 from Cheeseness/cheese-ball

Add cheese-ball.
Merge pull request #76 from fwp/ball-alpha

Use general alpha-test functionality for ball tests