Draw calls DrawMask with a nil mask.

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.

Over specifies ``(src in mask) over dst''.

Src specifies ``src in mask''.

A draw.Image is an image.Image with a Set method to change a single pixel.

Op is a Porter-Duff compositing operator.