PossiblyAtDestination checks if we have (probably) arrived in three ways:
- if we are a formation member: is the formation controller is arrived
- if we aren't, and we are not following a moving entity, assume out of waypoint == at destination
- if we aren't, and we are following a moving entity, check if need to move again.
This logic is weird and a bit broken, and instead we should use range checks from D918.
It also requires us to be moving to make sense, which prevents removing m_State.
NB: this is one of the very few dangerous patches of my UnitMotion rewrite - this //**needs**// to be handled correctly for movement to look good. I intend to use the pathfinding integration test map for this.
Also this will conflict with the CPP change I missed in D1865 so need to be careful there too.