title: How to write to zbuffer only with three.js
body: <p>I'm trying to use thee.js to only update the zbuffer (I'm using preserveDrawingBuffer to create a trace effect).
  However I can't find any way to only write to the zbuffer with the standard materials, so far I've tried:</p>
  
  <ul>
  <li>setting the material's <code>visible</code> to false, which stops the object rendering.</li>
  <li>setting the material's <code>opacity</code> to 0.0, which means nothing gets rendered.</li>
  </ul>
  
  <p>Is there a 'standard' way of doing this, or do I need to use a custom fragment shader?</p>