Codechange: pass "ground vehicle" to GetTileSlopeZ since for tunnel/bridges there are two states

Previously it checked the position in non-driving direction to "guess" whether
a ground vehicle was using the function, so on tunnels/bridges it could either
return the Z of the (virtual) ground compared to the Z of the path the vehicle
would take.

src/void_cmd.cpp

@@ -25,7 +25,7 @@ static void DrawTile_Void(TileInfo *ti)
 }
 
 
-static int GetSlopePixelZ_Void(TileIndex tile, uint x, uint y)
+static int GetSlopePixelZ_Void(TileIndex tile, uint x, uint y, bool ground_vehicle)
 {
 	/* This function may be called on tiles outside the map, don't assume
 	 * that 'tile' is a valid tile index. See GetSlopePixelZOutsideMap. */