draw: tweak the YCbCr to RGBA conversion formula again.

This is the golang.org/x equivalent of the golang.org/cl/36732 change to
the standard library.

Change-Id: I71c09a72e24c8e1c013769a6fed8f9a031724d9d
Reviewed-on: https://go-review.googlesource.com/36733
Run-TryBot: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
19 files changed