Maybe use smart constructor for Ray

[rrruko]

It might be useful to write an interface that ensures that rays always have a unit-length direction.

This would eliminate the need to normalize ray direction in places like intersectTri and generally be more correct since a ray's direction doesn't conceptually have a length at all. On the other hand, this would cause every single ray that is cast to get normalized exactly once, as opposed to a possibly zero indefinite number of times like we have now, which might hurt run time a lot.

Potential problem: what should happen if somebody tries to build a ray with a direction of length 0? Throwing an exception is kind of a lame solution.