New Butteraugli from Pik

[TiKevin83]

I noticed that there's new guetzli and butteraugli code in the pik repository so I attempted to fork this repository and backport the relevant Pik code. Seems to work properly and retain detail better than current, but someone who's written C and/or is more familiar with the project should probably check it over before anything like a pull request happens. Compressing towards butteraugli score still produces some strange results at high DPI for which I'll log a separate issue.

https://github.com/TiKevin83/guetzli/tree/ForRemote

[jyrkialakuijala]

I have a plan to do this within a few weeks. Guetzli is a practical program seeing actual production use, and updating guetzli requires some manual verification work which makes updating less frequent than something that is still in research phase (like pik).

I'm very thankful for any anomaly or strange/unwanted results reports, no matter how subtle the errors are.