fastApproximateAtan2
Computes a fast approximation of Atan2(x, y) for arbitrary input scalars.
Range reduction math based on nvidia's cg reference implementation: http://developer.download.nvidia.com/cg/atan2.html
Return
An approximation of atan2(x, y)
Parameters
x
An input number that isn't zero if y is zero.
y
An input number that isn't zero if x is zero.