image/draw: remove some bounds checks from DrawYCbCr

image/draw: remove some bounds checks from DrawYCbCr

It’d be nicer to write just

_ = dpix[x+3]

but the compiler isn’t able to reason about offsets
from symbols (yet).

image/draw benchmark:

YCbCr-8   722µs ± 3%   682µs ± 3%  -5.54%  (p=0.000 n=50+50)

Change-Id: Ia1e399496ed87c282bf0f9ca56c0b2d4948a0df9
Reviewed-on: https://go-review.googlesource.com/22146
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>