code: func newProjector(dst image.Rectangle, src image.Rectangle) *projector {
  	return &projector{
  		rx: float64(dst.Dx()) / float64(src.Dx()),
  		ry: float64(dst.Dy()) / float64(src.Dy()),
  		dx: float64(dst.Min.X - src.Min.X),
  		dy: float64(dst.Min.Y - src.Min.Y),
  		r:  dst,
  	}
  }
docs: // newProjector creates a Projector with source src and target dst.