image/color: improve speed of RGBA methods

image/color: improve speed of RGBA methods

Apply the optimizations added to color conversion functions in
https://go-review.googlesource.com/#/c/21910/ to the RGBA methods.

YCbCrToRGBA/0-4      6.32ns ± 3%  6.58ns ± 2%   +4.15%  (p=0.000 n=20+19)
YCbCrToRGBA/128-4    8.02ns ± 2%  5.89ns ± 2%  -26.57%  (p=0.000 n=20+19)
YCbCrToRGBA/255-4    8.06ns ± 2%  6.59ns ± 3%  -18.18%  (p=0.000 n=20+20)
NYCbCrAToRGBA/0-4    8.71ns ± 2%  8.78ns ± 2%   +0.86%  (p=0.036 n=19+20)
NYCbCrAToRGBA/128-4  10.3ns ± 4%   7.9ns ± 2%  -23.44%  (p=0.000 n=20+20)
NYCbCrAToRGBA/255-4  9.64ns ± 2%  8.79ns ± 3%   -8.80%  (p=0.000 n=20+20)

Fixes: #15260
Change-Id: I225efdf74603e8d2b4f063054f7baee7a5029de6
Reviewed-on: https://go-review.googlesource.com/31773
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>