Конвертировать ImageBitmap в Blob

fetch('https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png')
  .then(r => r.blob()) // yes.. from a Blob to a Blob...
// ImageBitmap generation  
  .then(createImageBitmap)
// to Blob
  .then(img => {
    console.log(img); // ImageBitmap
    return new Promise(res => {
      // create a canvas
      const canvas = document.createElement('canvas');
      // resize it to the size of our ImageBitmap
      canvas.width = img.width;
      canvas.height = img.height;
      // try to get a bitmaprenderer context
      let ctx = canvas.getContext('bitmaprenderer');
      if(ctx) {
        // transfer the ImageBitmap to it
        ctx.transferFromImageBitmap(img);
      }
      else {
        // in case someone supports createImageBitmap only
        // twice in memory...
        canvas.getContext('2d').drawImage(img,0,0);
      }
      // get it back as a Blob
      canvas.toBlob(res);
    });
  })
  .then(blob => {
    console.log(blob); // Blob
    var img = document.body.appendChild(new Image());
    img.src = URL.createObjectURL(blob);
  });