title: Fabric Js Image blend Filter issue
body: <p>When I try to apply a blend filter to an image in fabric JS I am getting some strange behavior. When I apply the filter, it just whites out the image rather than adding the specified color. Can anyone shed some light on what I am doing wrong? Below is the code and a link to a JS fiddle where I try to add the filter.
  Thanks
  <a href="http://jsfiddle.net/ahalbleib/bdofdbqg/2/" rel="nofollow">http://jsfiddle.net/ahalbleib/bdofdbqg/2/</a></p>
  
  <pre><code>function init() {
      var canvas = this.__canvas = new fabric.Canvas('c1', {
          hoverCursor: 'pointer'
      });
  
      var urls = ['https://dl.dropboxusercontent.com/s/ix6mvv49wnx226a/Central-Richmon_clipped_rev_1.png?dl=0',
          'https://dl.dropboxusercontent.com/s/jjp2l0kgdw8iitb/Laurel-Heights.png?dl=0',
          'https://dl.dropboxusercontent.com/s/wdk02w40z1466g5/LoneMountain.png?dl=0',
          'https://dl.dropboxusercontent.com/s/t6tnptndu2k22xr/OuterRichmond.png?dl=0',
          'https://dl.dropboxusercontent.com/s/tv4rhwjc0nw35iz/Presidio-Heights.png?dl=0',
          'https://dl.dropboxusercontent.com/s/ttbf390w2vdx4id/Inner-richmond.png?dl=0'];
      for (var i = 0; i &lt; urls.length; i++) {
          fabric.Image.fromURL(urls[i], function (img) {
  
              img.perPixelTargetFind = true;
              img.targetFindTolerance = 4;
              img.hasControls = img.hasBorders = false;
              img.filters.push(filter);
              var filter = new fabric.Image.filters.Blend({
                  image: img
              });
              filter.color = '#00f900';
              filter.mode = 'add';
              img.filters.push(filter);
              canvas.add(img).renderAll();
              img.applyFilters(canvas.renderAll.bind(canvas));
          }, {
              crossOrigin: 'Anonymous'
          });
      }
  
      canvas.findTarget = (function (originalFn) {
          return function () {
              var target = originalFn.apply(this, arguments);
              if (target) {
                  if (this._hoveredTarget !== target) {
                      canvas.fire('object:over', {
                          target: target
                      });
                      if (this._hoveredTarget) {
                          canvas.fire('object:out', {
                              target: this._hoveredTarget
                          });
                      }
                      this._hoveredTarget = target;
                  }
              } else if (this._hoveredTarget) {
                  canvas.fire('object:out', {
                      target: this._hoveredTarget
                  });
                  this._hoveredTarget = null;
              }
              return target;
          };
      })(canvas.findTarget);
  }
  
  init();
  </code></pre>