go-butteraugli

This is a direct port of the butteraugli (https://github.com/google/butteraugli) library for measuring the psychovisual similarity between two images.

This port was mostly a line-by-line translation from c++ to go.  The key difference is that float64 was used for both double and float.  This allowed for simpler code and did not affect the resulting diff value.

Example diff value:
butteraugli:    1.071696
go-butteraugli: 1.0716956373428894

This library is currently in parity with commit https://github.com/google/butteraugli/commit/88897aba.  Since 88897aba the authors have chosen to roll back roughly 1000 changelines (https://github.com/google/butteraugli/compare/26d3d9b8110499fa767673c1f713eacdf306aba5...master).  I'm going to wait until the library stabilizes to update this port.  Until then this port is out of parity and is producing very different numbers.

Tools:
A command-line tool for measuring the butteraugli diff value between two images is included in `cmd/compare_images`.  It serves as an example for how to use this library.

Contributing:

Pull requests are welcome that fix bugs or port updates to the butteraugli library to this code.  Changes that make the code more idiomatic to the go language are not welcome.