Another fix for some White/Black Dots

Unfortunately the clamping in the halton output does not fully solve the white dots in the enzo car demo scene. I have now black NaN dots where I had before the halton clamping.

This fix ensures the sqrt never tries to operate on negative values. It also simplifies calculation when s1=1.0 returning a trivial value and avoiding all the calculations in that particular case.
Merge pull request #3 from DavidBluecame/experimental-dt

Another fix for White Dots