code: func (a Affine) CenterFit(dst, src image.Rectangle) Affine {
  	dx := float64(dst.Min.X) + float64(dst.Dx())/2
  	dy := float64(dst.Min.Y) + float64(dst.Dy())/2
  	sx := float64(src.Min.X) + float64(src.Dx())/2
  	sy := float64(src.Min.Y) + float64(src.Dy())/2
  	return I.Translate(-sx, -sy).Mul(a).Translate(dx, dy)
  }
docs: // CenterFit produces the affine transform, centered around the rectangles.
  // It is equivalent to
  //   I.Translate(-<center of src>).Mul(a).Translate(<center of dst>)