Fix problem with floating avatar by passing avatar size information
to the physics engine.

This operation was accidentally removed while in the middle of improving
the computation of the avatar height. This is a temp fix until the real
solution is added.
BulletSim: make avatar physical shape to be a rectangle rather than
a capsule. Set the default to be the rectangle shape and adjust the
parameters in OpenSimDefaults.ini for the new shape.

The rectangle shape will perform better and avatar height can be
computed more accurately.