make megaclear 100x faster by not calling vrwho

Instead, we set an environment variable (ugly, I know) to indicate if we've built a VR world, then only clear VR if that variable is set.

This could potentially screw over someone who was constructing VRworlds in some way other than RigidBodyWRLVisualizer and expecting megaclear to clear them, but:
  
  * we have no evidence that such people exist, AND
  * megaclear in its current form _does not actually clear vrworld figures anyway_. We had to add the `vrclose` command to megaclear to get them to go away.