message: opencv.js: fix the loading issue of opencv.js(WASM) in face recognition
diff: <html>
  <head>
  - <script async src="../../opencv.js" type="text/javascript"></script>
  <script src="../../utils.js" type="text/javascript"></script>
  <script type='text/javascript'>
  var netDet = undefined, netRecogn = undefined;
  var persons = {};
  +var utils = new Utils('');
  //! [Run face detection model]
  function detectFaces(img) {
  @@ -68,7 +68,6 @@ function recognize(face) {
  //! [Recognize]
  function loadModels(callback) {
  - var utils = new Utils('');
  var proto = 'https://raw.githubusercontent.com/opencv/opencv/3.4/samples/dnn/face_detector/deploy.prototxt';
  var weights = 'https://raw.githubusercontent.com/opencv/opencv_3rdparty/dnn_samples_face_detector_20180205_fp16/res10_300x300_ssd_iter_140000_fp16.caffemodel';
  var recognModel = 'https://raw.githubusercontent.com/pyannote/pyannote-data/master/openface.nn4.small2.v1.t7';
  @@ -186,11 +185,16 @@ function main() {
  document.getElementById('startStopButton').disabled = false;
  };
  +
  +// Load opencv.js
  +utils.loadOpenCv(() => {
  + main();
  +});
  </script>
  </head>
  -<body onload="main()">
  +<body>
  <button id="startStopButton" type="button" disabled="true">Start</button>
  <div id="status"></div>
  <canvas id="output" width=640 height=480 style="max-width: 100%"></canvas>
  