Our approximation for log2() linearly interpolates between powers of
two. This is not accurate enough for some use cases. Squaring the input
effectively doubles the number of piecewise linear segments.

Bug swiftshader:71

Change-Id: Ie7a1da6b93cb5fbed018b61cf8510393964b5aa4
Reviewed-on: https://swiftshader-review.googlesource.com/10450Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>