code: func (im *Image) For(each func(x, y int) BGRA) {
  	for x := im.Rect.Min.X; x < im.Rect.Max.X; x++ {
  		for y := im.Rect.Min.Y; y < im.Rect.Max.Y; y++ {
  			im.SetBGRA(x, y, each(x, y))
  		}
  	}
  }
docs: // For transforms every pixel color to the color returned by 'each' given
  // an (x, y) position.