code: func Paste(background, img image.Image, pos image.Point) *image.NRGBA {
  	dst := Clone(background)
  	pos = pos.Sub(background.Bounds().Min)
  	pasteRect := image.Rectangle{Min: pos, Max: pos.Add(img.Bounds().Size())}
  	interRect := pasteRect.Intersect(dst.Bounds())
  	if interRect.Empty() {
  		return dst
  	}
  	src := newScanner(img)
  	parallel(interRect.Min.Y, interRect.Max.Y, func(ys <-chan int) {
  		for y := range ys {
  			x1 := interRect.Min.X - pasteRect.Min.X
  			x2 := interRect.Max.X - pasteRect.Min.X
  			y1 := y - pasteRect.Min.Y
  			y2 := y1 + 1
  			i1 := y*dst.Stride + interRect.Min.X*4
  			i2 := i1 + interRect.Dx()*4
  			src.scan(x1, y1, x2, y2, dst.Pix[i1:i2])
  		}
  	})
  	return dst
  }
docs: // Paste pastes the img image to the background image at the specified position and returns the combined image.