BulletSim: implement the SL bug where VEHICLE_HOVER_UP_ONLY disables
the vehicle buoyancy if the vehicle is above its hover height.

This is a known misfeature of this vehicle flag which has been accepted
since it would break too many implementations. The problem is noticed
when creating a jetski-like vehicle that jumps over sand bars. A boat
normally is configured with neutral buoyancy and hovering at water
height. When it jumps the sandbar, it needs to have gravity applied
to get back to water level.