title: Adding elem in canvas removes background and renders itself on a white canvas
body: <p>Adding any fabric js element like text or any shape in canvas removes background which i have set earlier to my canvas and object renders itself on a white canvas. I want to load object on my canvas having background.</p>
  
  <p>I tried renderAll() and bind() but it doesn't help at all.</p>
  
  <p>Here c is another canvas and window.canvas is another one, I'm setting c as background of window.canvas. Everything is fine here</p>
  
  <pre><code>var ctx = window.canvas.getContext("2d");
  var background = new Image();
  background.src = c.toDataURL("image/png", 1.0);
  
  background.onload = function(){
  ctx.drawImage(background,0,0,this.naturalWidth,this.naturalHeight,0,0,window.canvas.width,window.canvas.height);
  }
  
  window.canvas.setHeight(c.height);
  window.canvas.setWidth(c.width);
  window.canvas.renderAll();
  </code></pre>
  
  <p>Issue when i draw this shape or any other fabric element it loads on a white canvas instead on my old canvas which is window.canvas with some background</p>
  
  <pre><code>shapCirEl.onclick = function() {
          var cir = new fabric.Circle({
              radius: 60,
              fill: 'red',
              top: 100,
              left: 100
          });
          cir.set({
              borderColor: 'black',
              cornerColor: 'black',
              cornerSize: 6,
              transparentCorners: false,
              hasRotatingPoint: false
          });
          window.canvas.add(cir);
          window.canvas.setActiveObject(cir);
      };
  </code></pre>
  
  <p>When i draw this shape or any other fabric element it loads on a white canvas instead on my old canvas which is window.canvas with some background</p>