Players can noclip out of terrain and invisible walls

Using knockback + shotgun (and others), it is possible to propel yourself fast enough to trick the physics engine.

Our killboxes don't extend far enough to catch all players, and the invisible walls don't work properly.

We should consider enabling [continuous collision detection](https://docs.unity3d.com/Manual/ContinuousCollisionDetection.html) on major boundaries like terrain and invisible walls, which should help prevent these issues to some extent.