fix sum and sumsquare calculation: for the non FFT algorithm sum was...

fix sum and sumsquare calculation: for the non FFT algorithm sum was calculated twice before (stupid mistake), for the FFT algorithm there is actually a dependency for the sumsquare calculation on whether NumberPixels is odd or even. Although, the effect is very small for normal maps since high order fourier coefficients are small then. Anyhow, should be correct now