message: Fix type inconsistency in planetSelectionInvalid
diff: using namespace NAS2D;
  std::size_t planetSelection;
  -constexpr std::size_t planetSelectionInvalid = std::numeric_limits<int>::max();
  +constexpr std::size_t planetSelectionInvalid = std::numeric_limits<std::size_t>::max();
  static Font* FONT = nullptr;
  static Font* FONT_BOLD = nullptr;
  