This is just a copy/paste/rename in anticipation of automatically
generating fast-path code for specific dst-image types and src-image
types, a la the standard library's image/draw package.
Change-Id: If6385b347cb5e1d134b0ce841ef38ed4450a6fef
Reviewed-on: https://go-review.googlesource.com/5252
Reviewed-by: David Symonds <dsymonds@golang.org>
