refactor: convert engine to a singleton

src/gamestate.hpp

@@ -39,4 +39,6 @@ private:
     double average = 0;
 };
 
+inline std::unique_ptr<GameEngine> g_pEngine;
+
 #endif

src/main.cpp

@@ -1,8 +1,9 @@
 #include "gamestate.hpp"
+#include <memory>
 
 int main(void)
 {
-    GameEngine game;
-    game.Start();
+    g_pEngine = std::make_unique<GameEngine>();
+    g_pEngine->Start();
     return 0;
 }