Handle window close cleanly and show exit popup selection

src/audio/Audio.h

@@ -72,6 +72,7 @@ private:
     std::mutex tracksMutex;
     std::mutex pendingTracksMutex;
     std::atomic<bool> loadingComplete{false};
+    std::atomic<bool> loadingAbort{false};
     std::atomic<int> loadedCount{0};
 
     // SFX mixing support