title: Three.js: render to texture (RenderTarget) alpha not working
body: <p>I am trying to render to a texture with alpha and render that texture on top of a scene. The texture's background is not transparent though, the clear color is rendered over the scene.</p>
  
  <p>I created the renderer with alpha:true,
  the clear color has alpha 0,
  And the texture's format is RGBA</p>
  
  <p>What am I missing?</p>
  
  <pre><code>//creating the texture
  rtTexture = new THREE.WebGLRenderTarget( window.innerWidth, 
      window.innerHeight, { minFilter: THREE.LinearFilter, 
      magFilter: THREE.NearestFilter, format: THREE.RGBAFormat } );
  
  //creating renderer
  renderer = new THREE.WebGLRenderer({ alpha: true });
  renderer.setSize( window.innerWidth, window.innerHeight );
  renderer.autoClear = false;
  
  //textured quad
  var quad = new THREE.PlaneBufferGeometry(camSize,camSize/aspect);
  var mat = new THREE.MeshBasicMaterial({map:rtTexture,
      depthWrite:false, depthTest: false});
  var quadOb = new THREE.Mesh(quad, mat);
  
  //rendering
  renderer.setClearColor(0x000000, 0);
  renderer.render(sceneTorus, camera, rtTexture, true);
  renderer.render(sceneTet, cameraRTT, null, true);
  renderer.render(sceneQuad, cameraRTT, null, false);
  </code></pre>