code: func (captcha *CaptchaImage) DrawBorder(borderColor color.RGBA) *CaptchaImage {
  	if captcha.Error != nil {
  		return captcha
  	}
  	for x := 0; x < captcha.width; x++ {
  		captcha.nrgba.Set(x, 0, borderColor)
  		captcha.nrgba.Set(x, captcha.height-1, borderColor)
  	}
  	for y := 0; y < captcha.height; y++ {
  		captcha.nrgba.Set(0, y, borderColor)
  		captcha.nrgba.Set(captcha.width-1, y, borderColor)
  	}
  	return captcha
  }
docs: //画边框.