某些浏览器没有canvas.toBlob 方法的解决方案

var dataURLtoBlob = require('blueimp-canvas-to-blob');

// 80x60px GIF image (color black, base64 data):
var b64Data = 'R0lGODdhUAA8AIABAAAAAP///ywAAAAAUAA8AAACS4SPqcvtD6' +
        'OctNqLs968+w+G4kiW5omm6sq27gvH8kzX9o3n+s73/g8MCofE' +
        'ovGITCqXzKbzCY1Kp9Sq9YrNarfcrvcLDovH5PKsAAA7',
    imageUrl = 'data:image/gif;base64,' + b64Data,
    blob = dataURLtoBlob(imageURL);