Codechange: Use std::tuple for slope functions with two return values

src/road.cpp

@@ -35,7 +35,7 @@ static bool IsPossibleCrossing(const TileIndex tile, Axis ax)
 	return (IsTileType(tile, MP_RAILWAY) &&
 		GetRailTileType(tile) == RAIL_TILE_NORMAL &&
 		GetTrackBits(tile) == (ax == AXIS_X ? TRACK_BIT_Y : TRACK_BIT_X) &&
-		GetFoundationSlope(tile) == SLOPE_FLAT);
+		std::get<0>(GetFoundationSlope(tile)) == SLOPE_FLAT);
 }
 
 /**