draw: tweak the YCbCr to RGBA conversion formula.

This is the golang.org/x equivalent of the golang.org/cl/12220 change to
the standard library.

Change-Id: I220d1398902dec374472ef5b08db8501607cc971
Reviewed-on: https://go-review.googlesource.com/12222
Reviewed-by: Rob Pike <r@golang.org>
6 files changed