With Linux/8g on a 2006 Mac Mini (1.66 GHz Intel Core Duo,
2KB L1, 2MB L2, 2G main memory), GOMAXPROCS unset:

start:
png.BenchmarkEncodePaletted	      50	  44772820 ns/op
png.BenchmarkEncodeRGBOpaque	      10	 208395900 ns/op
png.BenchmarkEncodeRGBA		       5	 331088000 ns/op

remove interface method calls:
png.BenchmarkEncodePaletted	      50	  44722880 ns/op
png.BenchmarkEncodeRGBOpaque	      10	 139042600 ns/op
png.BenchmarkEncodeRGBA		       5	 334033600 ns/op

flate inline min/max():
png.BenchmarkEncodePaletted	      50	  40631180 ns/op
png.BenchmarkEncodeRGBOpaque	      10	 124894900 ns/op
png.BenchmarkEncodeRGBA		       5	 312099000 ns/op

after adler change:
png.BenchmarkEncodePaletted	      50	  40181760 ns/op
png.BenchmarkEncodeRGBOpaque	      20	 121781950 ns/op
png.BenchmarkEncodeRGBA		       5	 313890800 ns/op

In comparison to 121 ms on this 2006 machine, on my
Core2 Duo 2.66 GHz laptop, the final BenchmarkEncodeRGBOpaque
runs in 27 ms. (these are all for 640x480 images)

R=nigeltao, rsc, r
CC=golang-dev
https://golang.org/cl/4432077