PolyObject fixes backport

This branch backports some recently made fixes to PolyObject physics issues we were aware of in 2.2.

The following bugs are fixed:

• When riding a PolyObject platform following waypoints, and you decide to walk off the edge of the platform, you immediately teleport to the ground. This was linked to an earlier MR attempting to allow PolyObjects to crush players vertically IIRC, and my fix for this probably makes !422 (closed) redundant.
• If the PolyObject's outer control sector is not the same as that which the PolyObject is actually physically inside, it is possible to "collide" with thin air below or above the PolyObject, depending on the heights of the sectors involved.