title: Three.js: Trouble with dynamic geometry and merging
body: <p>I want to display a lot of cubes. Their position is calculated during runtime, one more per frame. Just doing <code>scene.add(cube)</code> each time works but is too slow. So I want to merge their geometries each time. But when I try to add the merged geometry to the scene, it still shows the old version! I tried all kinds of <code>.updateMatrix()</code> and <code>.verticesNeedUpdate=true</code> things.</p>
  
  <p>Interestingly the problem only comes up if I try to change things I have already added to the scene. It does work if I just do my merging in the background and add the result to the scene at some point! But After it has been added, merging it and adding the result still displays the old version. I've tried removing the Object from the scene first, but that doesn't change anything.</p>