The current iteration can decode 8-bit images in
grayscale, paletted, RGB, RGBA and NRGBA mode. LZW compression
is implemented but does not work on my test images.
Deflate (i.e. zlib) compression with or without a horizontal
predictor is supported.

R=nigeltao, nigeltao_gnome
CC=golang-dev, mpl
https://golang.org/cl/4240051