Sphere sweep

[Leon Pennington]

Done the changes and committed them. I've twisted the sphere sweep in all 
sorts of directions and the pinching seems to have disappeared.

Sorry it took a while but this was a tricky one to fix. I eventually replaced 
the orthogonal of the direction with a unit length cross product of the 
direction vector by the orthogonal of the total vector of the segment. Not an 
easy solution to come up with but it works.

-- 
Leon Pennington

"The most likely way for the world to be destroyed, most experts agree, is by 
accident. That's where we come in; we're computer professionals. We cause 
accidents." 
 Nathaniel Borenstein.