title: Transparent shader pass with effect composer
body: <p>I'm trying to render a scene and then have a half-transparent post-processing effect rendered on top of it. I'd like to make it work with effect composer so I can later use more passes easily.</p>
  
  <p>My basic structure is this:</p>
  
  <pre><code>composer = new Three.EffectComposer renderer
  composer.addPass renderPass
  composer.addPass transparentPass
  composer.addPass copyPass
  </code></pre>
  
  <p>copyPass has renderToScreen = true, transparentPass is my custom fragment shader with each pixel's alpha is set to 0.5 and loaded with ShaderPass.</p>
  
  <p>I've tried blending, depthTest and transparent for the shaderMaterial.
  Also tried alpha and premultipliedAlpha options for renderer, autoClear off.</p>
  
  <p>I'm getting either the scene from renderPass with nothing else or the transparentPass on any renderer.clearColor I choose. Not the composite of both. What do I need to set to make this work?
  Thank you.</p>
  
  <p>EDIT:</p>
  
  <p>Here is a codepen example:
  <a href="http://codepen.io/Eskel/pen/PzaOWb?editors=0010" rel="nofollow">http://codepen.io/Eskel/pen/PzaOWb?editors=0010</a></p>
  
  <p>And the code of my shader:
  <a href="http://eskel.cz/js/three79/RGBAShader.js" rel="nofollow">http://eskel.cz/js/three79/RGBAShader.js</a></p>
  
  <p>It shows all white (and not a rotating cube underneath) even though all pixels are rendered with alpha channel set to 0.5. Should I mix it with the tDiffuse render target in the shader or is there a way to make the shaderPass transparent?</p>