Use IsGhost() to access ghost state

Added set and clear ghost to further prevent direct access to flags

src/openrct2/ride/TrackDesign.cpp

@@ -2129,7 +2129,7 @@ static money32 place_maze_design(uint8_t flags, Ride* ride, uint16_t mazeEntry,
         tileElement->AsTrack()->SetMazeEntry(mazeEntry);
         if (flags & GAME_COMMAND_FLAG_GHOST)
         {
-            tileElement->flags |= TILE_ELEMENT_FLAG_GHOST;
+            tileElement->SetGhost();
         }
 
         map_invalidate_element(fx, fy, tileElement);