The existing tracing routine for building the PVS was rather simple compared to the main portal tracing, and could not correctly cope with paths that went through multiple portals from room A to B, and as a result would sometimes miss room entries in the PVS resulting in too many culled rooms in these circumstances.
This PR adds an improved function that can cope with entering a room multiple times during a trace. As a result it has to take care of portal directions (to prevent going back on itself) in a similar, but not identical way to the main portal tracing routine, and internal rooms, to prevent recursive loops.
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