[css-transforms-1] SLERP edge case behaviour

Hi list,

Here's how we currently define quaternion SLERPing during interpolation:
http://dev.w3.org/csswg/css-transforms/#interpolation-of-decomposed-3d-matrix-values

When quatA = [0, 1, 0, 0] and quatB = [0, -1, 0, 0], this has the
unfortunate side effect of setting product to -1, which means the
definition of 'w' has 0 in the denominator.

What do?

Cheers,
    -Shane