title: SkeletonHelper is not rendered
body: <p>I am trying to display a Three JS Skeleton using SkeletonHelper.</p>
  
  <p>According to this <a href="https://stackoverflow.com/questions/52231144/using-a-skeleton-with-skeletonhelper">answer</a>, SkeletonHelper() takes the root bone. I have done just that. But it is not being displayed. Why is this the case?</p>
  
  <pre><code>var bones = []
  var shoulder = new THREE.Bone();
  var elbow = new THREE.Bone();
  var hand = new THREE.Bone();
  
  shoulder.add(elbow);
  elbow.add(hand);
  
  bones.push(shoulder);
  bones.push(elbow);
  bones.push(hand);
  
  skeleton = new THREE.Skeleton(bones)
  
  skeleton_helper = new THREE.SkeletonHelper(shoulder)
  scene.add(skeleton_helper)
  }
  </code></pre>
  
  <p>Thanks for the help!</p>