Fabric JS图像遮罩如何处理？

var img01URL = 'http://fabricjs.com/assets/printio.png';
var img02URL = 'http://fabricjs.com/lib/pug.jpg';
var img03URL = 'http://fabricjs.com/assets/ladybug.png';
var img03URL = 'http://fabricjs.com/assets/ladybug.png';



    fabric.Image.prototype._render = function(ctx) {
      // custom clip code
      if (this.clipPath) {
        ctx.save();
        if (this.clipPath.fixed) {
          var retina = this.canvas.getRetinaScaling();
          ctx.setTransform(retina, 0, 0, retina, 0, 0);
          // to handle zoom
          ctx.transform.apply(ctx, this.canvas.viewportTransform);
          //
          this.clipPath.transform(ctx);
        }
        this.clipPath._render(ctx);
        ctx.restore();
        ctx.clip();
      }
      
      // end custom clip code
    
    
      var x = -this.width / 2, y = -this.height / 2, elementToDraw;

      if (this.isMoving === false && this.resizeFilter && this._needsResize()) {
        this._lastScaleX = this.scaleX;
        this._lastScaleY = this.scaleY;
        this.applyResizeFilters();
      }
      elementToDraw = this._element;
      elementToDraw && ctx.drawImage(elementToDraw,
                                     0, 0, this.width, this.height,
                                     x, y, this.width, this.height);
      this._stroke(ctx);
      this._renderStroke(ctx);
    };

var canvas = new fabric.Canvas('c');
canvas.setZoom(0.5)
fabric.Image.fromURL(img01URL, function(oImg) {
    oImg.scale(.25);
    oImg.left = 10;
    oImg.top = 10;
    oImg.clipPath = new fabric.Circle({radius: 40, top: 50, left: 50, fixed: true, fill: '', stroke: '' });
    canvas.add(oImg);
    canvas.renderAll();
});

fabric.Image.fromURL(img02URL, function(oImg) {
    oImg.scale(.40);
    oImg.left = 180;
    oImg.top = 0;
    oImg.clipPath = new fabric.Path('M85.6,606.2c-13.2,54.5-3.9,95.7,23.3,130.7c27.2,35-3.1,55.2-25.7,66.1C60.7,814,52.2,821,50.6,836.5c-1.6,15.6,19.5,76.3,29.6,86.4c10.1,10.1,32.7,31.9,47.5,54.5c14.8,22.6,34.2,7.8,34.2,7.8c14,10.9,28,0,28,0c24.9,11.7,39.7-4.7,39.7-4.7c12.4-14.8-14-30.3-14-30.3c-16.3-28.8-28.8-5.4-33.5-11.7s-8.6-7-33.5-35.8c-24.9-28.8,39.7-19.5,62.2-24.9c22.6-5.4,65.4-34.2,65.4-34.2c0,34.2,11.7,28.8,28.8,46.7c17.1,17.9,24.9,29.6,47.5,38.9c22.6,9.3,33.5,7.8,53.7,21c20.2,13.2,62.2,10.9,62.2,10.9c18.7,6.2,36.6,0,36.6,0c45.1,0,26.5-15.6,10.1-36.6c-16.3-21-49-3.1-63.8-13.2c-14.8-10.1-51.4-25.7-70-36.6c-18.7-10.9,0-30.3,0-48.2c0-17.9,14-31.9,14-31.9h72.4c0,0,56-3.9,70.8,26.5c14.8,30.3,37.3,36.6,38.1,52.9c0.8,16.3-13.2,17.9-13.2,17.9c-31.1-8.6-31.9,41.2-31.9,41.2c38.1,50.6,112-21,112-21c85.6-7.8,79.4-133.8,79.4-133.8c17.1-12.4,44.4-45.1,62.2-74.7c17.9-29.6,68.5-52.1,113.6-30.3c45.1,21.8,52.9-14.8,52.9-14.8c15.6,2.3,20.2-17.9,20.2-17.9c20.2-22.6-15.6-28-16.3-84c-0.8-56-47.5-66.1-45.1-82.5c2.3-16.3,49.8-68.5,38.1-63.8c-10.2,4.1-53,25.3-63.7,30.7c-0.4-1.4-1.1-3.4-2.5-6.6c-6.2-14-74.7,30.3-74.7,30.3s-108.5,64.2-129.6,68.9c-21,4.7-18.7-9.3-44.3-7c-25.7,2.3-38.5,4.7-154.1-44.4c-115.6-49-326,29.8-326,29.8s-168.1-267.9-28-383.4C265.8,13,78.4-83.3,32.9,168.8C-12.6,420.9,98.9,551.7,85.6,606.2z',{top: 0, left: 180, fixed: true, fill: '', stroke: '', scaleX: 0.2, scaleY: 0.2 });
    canvas.add(oImg);
    canvas.renderAll();
});

#c {
    border:1px solid #ccc;
}

<script src="//cdnjs.cloudflare.com/ajax/libs/fabric.js/1.7.16/fabric.min.js"></script>
<canvas id="c" width="400" height="400"></canvas>