Snap to the ground after moving

Depends on two factors:
* End up close enough above to a walkable plane (it's within sMaxStep units
  down and is angled sMaxSlope or less)
* Started out on the ground without any upward movement

This also reduces the distance needed to be to the ground to 4 (from 10), and
ensures the actor is 2 units above the ground when on it. Downward force is
also removed when starting on the ground.
Increase step size to 15