The image/draw benchmark:
benchmark          old ns/op     new ns/op     delta
BenchmarkYCbCr     1198605       978647        -18.35%

Change-Id: Iacfc21e6f641ecb05adc00b3aec0048f1f43d265
Reviewed-on: https://go-review.googlesource.com/7952Reviewed-by: Rob Pike <r@golang.org>