code: func NewCaptchaImage(width int, height int, bgColor color.RGBA) *CaptchaImage {
  
  	m := image.NewNRGBA(image.Rect(0, 0, width, height))
  
  	draw.Draw(m, m.Bounds(), &image.Uniform{C: bgColor}, image.ZP, draw.Src)
  
  	return &CaptchaImage{
  		nrgba:  m,
  		height: height,
  		width:  width,
  	}
  }
docs: //新建一个图片对象