Recheck why we can't FindPath on the searchmap

We tried already, but that caused severe bugs as detailed in #2223. We're back to searching on the navigation map, but that's quite slower (and did nothing for #2208).

We do have a test in the suite that breaks if looking on the searchmap is reenabled (see `IsWalkableTo` comment in `FindPath`). Or the more complex bg1 ones from the end of the other bug. The task is to find out the core reason why the algorithm fails to find a proper path and the actor tries to walk through a wall.