Bug in Atwood Simulation

[yatna]

In the Atwood Simulation, the bodies are connected to a pulley via a thread. However, when the position of any body moves above the position of pulley mid-point, a thread a shown going from within the body from the pulley mid-point to the top of the body. This should not happen as the bodies are opaque. I have corrected this and now the simulation looks more realistic. I would like to send a PR. @croozeus @costinpapuc