BulletSim: fix exception when re-creating the terrain when loading an OAR file
BulletSim: add function to push avatar up when hitting stairs.
It looks like BulletSim and ODE rely on penetration correction to cause
the avatar to move up and thus allowing walking up stairs. Object
penetration was minimized for walking and flying (so one doesn't go
through walls) and this stopped stairs from working. This commit
introduces avatar movement code to check for collisions at the
feet while walking and attempts to raise the avatar for the steps.
Not yet perfect but movement is better.