The imageutil.DrawYCbCr function lives in an internal package because it
is needed by both the image/draw and image/jpeg packages, but it doesn't
seem right for one of those two to depend on the other.

It could eventually go into the image package, but that would require
committing to an API for the rest of Go 1.x.

Change-Id: I7b12555c970d86409365e99eef9360702aaffa30
Reviewed-on: https://go-review.googlesource.com/7925Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>