This fixes the lossy_extreme_probabilities.webp conformance test.
LGTM=pascal.massimino, r
R=r, pascal.massimino
CC=golang-codereviews
https://golang.org/cl/145600043
code.google.com/p/vp8-go
This is a straight copy, followed by import path updates.
LGTM=r
R=r, mirtchovski, oleku.konko
CC=golang-codereviews
https://golang.org/cl/79320043