Sphere intersection not correct

[athorwall]

https://github.com/rustgd/collision-rs/blob/master/src/volume/sphere.rs#L61 seems to be incorrect. Specifically the computation of tca:

let tca = l.dot(r.direction);

where tca is intended to be the length of the projection of l onto r.direction is incorrect - it needs to be divided by the magnitude of r.direction:

let tca = l.dot(r.direction) / r.direction.magnitude();

[athorwall]

I just noticed that the tests always normalize ray directions, which would address this - is it just always assumed that ray directions are normalized, through the whole library? If so it's unfortunate that it's possible to construct a non-normalized Ray.

[Rhuagh]

All intersection tests assume Ray direction is normalized, to avoid a hidden square root computation. Rays can be created without normalized direction because of the same reason.

We should probably document this behavior more clearly.

[kvark]

@Rhuagh we can also debug_assert on normalization in the code, where appropriate