title: How to add shader to THREE.Object3D loaded from OBJMTLLoader
body: <p>I have a model loaded using <a href="https://github.com/mrdoob/three.js/blob/master/examples/js/loaders/OBJMTLLoader.js" rel="nofollow"><code>THREE.OBJMTLLoader</code></a>.</p>
  
  <pre><code>var loader = new THREE.OBJMTLLoader();
  loader.addEventListener('load', function(event) {
      var mesh = event.content;
      scene.add(mesh);
  });
  loader.load('model/machine.obj', 'model/machine.mtl');
  </code></pre>
  
  <p>I need to apply a vertex and fragment shader to this model. How to do this?</p>