- GetSectorFloorZ/GetSectorCeilingZ tag=0 handling

Okay I reconsidered what to do and here's a pretty generic solution. With this patch GetSectorFloorZ and GetSectorCeilingZ consider tag=0 to mean 'any sector' and look up the sector at the coordinates given.

tag=0 is useless as it is because the function first gets the sector by tag first and then gets the coordinates, so it only works for single tagged sectors. This adds a generalized way of getting z by x and y at whatever point in the map.