message: Add default level of 0 for isValidPosition
  Doesn't matter if the default is 0, or mCurrentLevel, as either is
  guaranteed to be valid.
diff: @@ -33,7 +33,7 @@ public:
  TileMap& operator=(const TileMap&) = delete;
  ~TileMap() override;
  - bool isValidPosition(NAS2D::Point<int> position, int level) const;
  + bool isValidPosition(NAS2D::Point<int> position, int level = 0) const;
  Tile* getTile(NAS2D::Point<int> position, int level);
  Tile* getTile(NAS2D::Point<int> position) { return getTile(position, mCurrentDepth); }
  