title: Mesh position invalid after changing the vertice positions dynamically
body: <p>I try to create a mesh with PlaneGeometry. After I change the geometry's vertices dynamically the mesh.position and mesh.rotation is not correct afterwards. Is it possible to recompute these accordingly with the new vertices?</p>
  
  <p>I try to do this:</p>
  
  <pre><code>var geometry = new THREE.PlaneGeometry(500, 500);
  geometry.dynamic = true;
  var material = new THREE.MeshBasicMaterial({
              side: THREE.DoubleSide
  });
  var plane = new THREE.Mesh(geometry, material);
  scene.add(plane);
  
  geometry.vertices[0].x += 15;
  geometry.vertices[1].x += 15;
  geometry.vertices[2].x += 10;
  geometry.vertices[3].x += 10;
  geometry.verticesNeedUpdate = true;
  </code></pre>
  
  <p>Here plane.position is still Vector3(0, 0, 0). How can I update this to correct position?</p>