Fixes problems in LUV -> XYZ conversion when white point Y != 1

In the conversion for LUV to XYZ, the current math assumed that the Y value of the white point would always be 1. This meant that when you used XYZ -> [0, 100], a scale factor would be introduced during the transformation.

This ensures that XYZ values of any scale (including un-normalized values from a measurement device) can be converted without error.