(svn r1834) - Fix: NPF does not check the owner of its target, busses try to enter other players' depots. TODO

- Add: asserts to find the v->u.rail.track == 0 problem.
- Add: IsValidDepot(), IsValidTown(), IsValidSign(), IsValidVehicle(), IsValidStation()
- Add: GetTileOwner(), IsTileOwner()
- Codechange: Replaced IsShipDepotTile(), IsTrainDepotTile(), IsRoadDepotTile() by IsTileDepotType().
- Codechange: typedeffed the MAP_OWNERS as Owner. Should be used as variable type.
- Codechange: Replaced a few uint by TileIndex.

station.h

@@ -272,6 +272,14 @@ static inline bool IsRoadStationTile(uint tile) {
 	return IsTileType(tile, MP_STATION) && IS_BYTE_INSIDE(_map5[tile], 0x43, 0x4B);
 }
 
+/**
+ * Check if a station really exists.
+ */
+static inline bool IsValidStation(Station* station)
+{
+	return station->xy != 0; /* XXX: Replace by INVALID_TILE someday */
+}
+
 /* Get's the direction the station exit points towards. Ie, returns 0 for a
  * station with the exit NE. */
 static inline byte GetRoadStationDir(uint tile) {