Several improvements to the physics of objects

Taken from WinMBF, "torque" is now applied to objects hanging off of
ledges "which avoids anomalies such as lifeless objects hanging more
than halfway off of ledges, and allows objects to roll off of the edges
of moving lifts, or to slide up and then back down stairs, or to fall
into a ditch". Also, falling objects are prevented from going up too
many steps.