Grayscale PNG and JPEG images are not uncommon. We should have a fast path.

Also add a benchmark for the recently added CMYK fast path.

benchmark                    old ns/op     new ns/op     delta
BenchmarkGray                13960348      324152        -97.68%

Change-Id: I72b5838c8c3d1f2d0a4536a848e020e80b10c0f7
Reviewed-on: https://go-review.googlesource.com/6237Reviewed-by: Rob Pike <r@golang.org>