image/draw: improve performances if mask is *image.Alpha

The existing DrawMask method is generic and is therefore calling the At().RGBA() method for every pixel of the mask and the source.
Do a specific implementation when the mask is *image.Alpha (which is
common) and use use the PixOffset method to increase performances.

name                old time/op  new time/op  delta
RGBA2-12            1.60ms ± 0%  1.13ms ± 1%  -29.16%  (p=0.008 n=5+5)
GenericMaskOver-12   915µs ± 4%   926µs ± 1%     ~     (p=0.190 n=5+4)
RGBA64Over-12       1.53ms ± 3%  1.21ms ± 2%  -20.74%  (p=0.008 n=5+5)
GrayOver-12         1.36ms ± 2%  1.01ms ± 7%  -26.27%  (p=0.008 n=5+5)

Fixes: #46395
Change-Id: Iaeaa8cfcc6a3fe93eb19b361f3bf076e41cac5b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/323749


Reviewed-by: Nigel Tao <nigeltao@golang.org>
Trust: Nigel Tao <nigeltao@golang.org>
Trust: Andrew Gerrand <adg@golang.org>
Run-TryBot: Nigel Tao <nigeltao@golang.org>