Teetering now supports slopes properly.

Behaves ALMOST as you'd expect. It gets the z position of the slope at the player coordinates when it comes to the sectorlist check (which is first), though, so there's a few oddities that are amplified with steep slopes:
* If the slope's sloping away from you at a steep angle, you might not be able to step down onto it, but you won't teeter (because it's at a step-down-able height if it extended to directly beneath you)
* If the slope's sloping towards you at a steep angle, you might end up in teetering frames when you're able to step down onto it (because it's NOT at a step-down-able height if it extended to directly beneath you)

HOWEVER, it would be pretty obnoxious to hold back code which is functionally superior in every way otherwise, and it doesn't really seem like there's a good way to get that checked tbph