Package draw provides image composition functions.
See "The Go image/draw package" for an introduction to this package: http://golang.org/doc/articles/image_draw.html
Draw calls DrawMask with a nil mask.
func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, op Op)
DrawMask aligns r.Min in dst with sp in src and mp in mask and then replaces the rectangle r in dst with the result of a Porter-Duff composition. A nil mask is treated as opaque.
A draw.Image is an image.Image with a Set method to change a single pixel.
type Op int
Op is a Porter-Duff compositing operator.