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>