refactor: move _game declaration to cpp file for better encapsulation

Author: ShingZhanho

src/game_level_ui.cpp

@@ -2,6 +2,8 @@
 #include <util/log.hpp>
 #include <ui/common.hpp>
 
+static core::Game* _game = nullptr;
+
 void gameLvl_configureGameOptions(core::GameOptions* options) {
     util::WriteToLog("Configuring game options...", "gameLvl_configureGameOptions()");
     gameLvl_gameOptions = options;

src/game_level_ui.hpp

@@ -8,7 +8,7 @@
 
 // ==== IMPORTANT ====
 // All variables/functions that will be used outside this file MUST begin with "gameLvl_"
-// All variables/functions that will be used only in this file MUST begin with "_"
+// All variables/functions that will be used only in this file MUST be defined in the .cpp file, not here.
 
 #include <core/game.hpp>
 
@@ -24,7 +24,5 @@ static std::atomic<bool> gameLvl_customMode = false;
 void gameLvl_mainGameLoop();
 // The score of the game.
 static int gameLvl_score = 0;
-// The private Game object.
-static core::Game* _game;
 
 #endif // _GAME_LEVEL_UI_HPP