image/png: optimize paeth some more.
filterPaeth takes []byte arguments instead of byte arguments,
which avoids some redudant computation of the previous pixel
in the inner loop.
Also eliminate a bounds check in decoding the up filter.
benchmark old ns/op new ns/op delta
BenchmarkDecodeGray
3139636 2812531 -10.42%
BenchmarkDecodeNRGBAGradient
12341520 10971680 -11.10%
BenchmarkDecodeNRGBAOpaque
10740780 9612455 -10.51%
BenchmarkDecodePaletted
1819535 1818913 -0.03%
BenchmarkDecodeRGB
8974695 8178070 -8.88%
R=rsc
CC=golang-dev
https://golang.org/cl/
6243061