Write to image.*.Pix directly in the case of RGB, RGBA and NRGBA
images. For the latter two, the file format matches the memory layout
so a simple copy can be used.

RGB image before/after:
tiff.BenchmarkDecoder	748137 ns/op (62.39 MB/s)	251256 ns/op (185.76 MB/s)	x3.0

NRGBA image before/after:
tiff.BenchmarkDecoder	775540 ns/op (80.12 MB/s)	116721 ns/op (532.34 MB/s)	x6.6

R=nigeltao
CC=golang-dev
https://golang.org/cl/4929046