code: func Mask(w http.ResponseWriter, req *http.Request) {
  	arg := func(s string) int { x, _ := strconv.Atoi(req.FormValue(s)); return x }
  	v := arg("v")
  	m := arg("m")
  	scale := arg("scale")
  	if scale == 0 {
  		scale = 8
  	}
  
  	w.Header().Set("Cache-Control", "public, max-age=3600")
  	w.Write(pngEncode(makeMask(req, req.FormValue("font"), arg("pt"), v, m, scale)))
  }
docs: // Mask handles a request for a single QR mask.