benchmark                        old ns/op     new ns/op     delta
BenchmarkPaeth                   5.06          6.02          +18.97%
BenchmarkDecodeGray              1010551       956911        -5.31%
BenchmarkDecodeNRGBAGradient     3877813       3754160       -3.19%
BenchmarkDecodeNRGBAOpaque       3194058       3079094       -3.60%
BenchmarkDecodePaletted          699243        700211        +0.14%
BenchmarkDecodeRGB               2835733       2692120       -5.06%
BenchmarkDecodeInterlacing       3651805       3563124       -2.43%
BenchmarkEncodeGray              4399183       4404113       +0.11%
BenchmarkEncodeNRGBOpaque        13323627      13306485      -0.13%
BenchmarkEncodeNRGBA             15840092      15751188      -0.56%
BenchmarkEncodePaletted          4396622       4404373       +0.18%
BenchmarkEncodeRGBOpaque         13320475      13279189      -0.31%
BenchmarkEncodeRGBA              36898392      36781002      -0.32%

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://golang.org/cl/117290043