Fixes #6293.

Image "testdata/benchRGB-interlace.png" was generated by opening "testdata/benchRGB.png" in the editor Gimp and saving it with interlacing enabled.

Benchmark:
BenchmarkDecodeRGB        	     500	   7014194 ns/op	  37.37 MB/s
ok  	pkg/image/png	4.657s

BenchmarkDecodeInterlacing	     100	  10623241 ns/op	  24.68 MB/s
ok  	pkg/image/png	1.339s

LGTM=nigeltao
R=nigeltao, andybons, matrixik
CC=golang-codereviews
https://golang.org/cl/102130044