canvas toBlob ，ie兼容

/* canvas-toBlob.js
 * A canvas.toBlob() implementation.
 * 2016-05-26
 * 
 * By Eli Grey, http://eligrey.com and Devin Samarin, https://github.com/eboyjr
 * License: MIT
 *   See https://github.com/eligrey/canvas-toBlob.js/blob/master/LICENSE.md
 */

/*global self */
/*jslint bitwise: true, regexp: true, confusion: true, es5: true, vars: true, white: true,
  plusplus: true */

/*! @source http://purl.eligrey.com/github/canvas-toBlob.js/blob/master/canvas-toBlob.js */

(function(view) {
"use strict";
var
      Uint8Array = view.Uint8Array
    , HTMLCanvasElement = view.HTMLCanvasElement
    , canvas_proto = HTMLCanvasElement && HTMLCanvasElement.prototype
    , is_base64_regex = /\s*;\s*base64\s*(?:;|$)/i
    , to_data_url = "toDataURL"
    , base64_ranks
    , decode_base64 = function(base64) {
        var
              len = base64.length
            , buffer = new Uint8Array(len / 4 * 3 | 0)
            , i = 0
            , outptr = 0
            , last = [0, 0]
            , state = 0
            , save = 0
            , rank
            , code
            , undef
        ;
        while (len--) {
            code = base64.charCodeAt(i++);
            rank = base64_ranks[code-43];
            if (rank !== 255 && rank !== undef) {
                last[1] = last[0];
                last[0] = code;
                save = (save << 6) | rank;
                state++;
                if (state === 4) {
                    buffer[outptr++] = save >>> 16;
                    if (last[1] !== 61 /* padding character */) {
                        buffer[outptr++] = save >>> 8;
                    }
                    if (last[0] !== 61 /* padding character */) {
                        buffer[outptr++] = save;
                    }
                    state = 0;
                }
            }
        }
        // 2/3 chance there‘s going to be some null bytes at the end, but that
        // doesn‘t really matter with most image formats.
        // If it somehow matters for you, truncate the buffer up outptr.
        return buffer;
    }
;
if (Uint8Array) {
    base64_ranks = new Uint8Array([
          62, -1, -1, -1, 63, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1
        , -1, -1,  0, -1, -1, -1,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9
        , 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25
        , -1, -1, -1, -1, -1, -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35
        , 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51
    ]);
}
if (HTMLCanvasElement && (!canvas_proto.toBlob || !canvas_proto.toBlobHD)) {
    if (!canvas_proto.toBlob)
    canvas_proto.toBlob = function(callback, type /*, ...args*/) {
          if (!type) {
            type = "image/png";
        } if (this.mozGetAsFile) {
            callback(this.mozGetAsFile("canvas", type));
            return;
        } if (this.msToBlob && /^\s*image\/png\s*(?:$|;)/i.test(type)) {
            callback(this.msToBlob());
            return;
        }

        var
              args = Array.prototype.slice.call(arguments, 1)
            , dataURI = this[to_data_url].apply(this, args)
            , header_end = dataURI.indexOf(",")
            , data = dataURI.substring(header_end + 1)
            , is_base64 = is_base64_regex.test(dataURI.substring(0, header_end))
            , blob
        ;
        if (Blob.fake) {
            // no reason to decode a data: URI that‘s just going to become a data URI again
            blob = new Blob
            if (is_base64) {
                blob.encoding = "base64";
            } else {
                blob.encoding = "URI";
            }
            blob.data = data;
            blob.size = data.length;
        } else if (Uint8Array) {
            if (is_base64) {
                blob = new Blob([decode_base64(data)], {type: type});
            } else {
                blob = new Blob([decodeURIComponent(data)], {type: type});
            }
        }
        callback(blob);
    };

    if (!canvas_proto.toBlobHD && canvas_proto.toDataURLHD) {
        canvas_proto.toBlobHD = function() {
            to_data_url = "toDataURLHD";
            var blob = this.toBlob();
            to_data_url = "toDataURL";
            return blob;
        }
    } else {
        canvas_proto.toBlobHD = canvas_proto.toBlob;
    }
}
}(typeof self !== "undefined" && self || typeof window !== "undefined" && window || this.content || this));