Small physics optimization

I noticed that convexSweepTest was being called a minimum of 2 times per frame for each actor, even if they are not moving. This change combines the sweep tests used for checking on ground status and for stepping down surfaces into one. Movement should still behave the same.

Each convexSweepTest takes about 0.1 ms on my machine, so this should give a small performance boost (roughly 2 ms in Balmora).