up to now, I mainly worked on speeding up SSIMU2 so there was a good margin of time to gain on butteraugli. Expect from 1.2x to 1.4x speedup
I also fixed vship when it received < -0.04 srgb values (I was not expecting negative inputs)
the downsample in butteraugli is now done after transition to linear RGB.
That is all for today!