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