Pause RGSS execution when moving into background on Android

Assuming that there is enough memory for mkxp to stay in the
background and that the OS doesn't kill the process, this should
allow smooth resuming after moving back into the foreground.

For now, EGL context loss is not handled.

src/eventthread.h

@@ -39,6 +39,7 @@
 struct RGSSThreadData;
 typedef struct ALCdevice_struct ALCdevice;
 struct SDL_Window;
+union SDL_Event;
 
 #define MAX_FINGERS 4
 
@@ -98,6 +99,8 @@ public:
 	void notifyFrame();
 
 private:
+	static int eventFilter(void *, SDL_Event*);
+
 	void resetInputStates();
 	void setFullscreen(SDL_Window *, bool mode);
 	void updateCursorState(bool inWindow);
@@ -174,6 +177,39 @@ private:
 	T current;
 };
 
+struct SyncPoint
+{
+	/* Used by eventFilter to control sleep/wakeup */
+	void haltThreads();
+	void resumeThreads();
+
+	/* Used by RGSS thread */
+	bool mainSyncLocked();
+	void waitMainSync();
+
+	/* Used by secondary (audio) threads */
+	void passSecondarySync();
+
+private:
+	struct Util
+	{
+		Util();
+		~Util();
+
+		void lock();
+		void unlock(bool multi);
+		void waitForUnlock();
+
+		AtomicFlag locked;
+		SDL_mutex *mut;
+		SDL_cond *cond;
+	};
+
+	Util mainSync;
+	Util reply;
+	Util secondSync;
+};
+
 struct RGSSThreadData
 {
 	/* Main thread sets this to request RGSS thread to terminate */
@@ -191,6 +227,7 @@ struct RGSSThreadData
 	EventThread *ethread;
 	UnidirMessage<Vec2i> windowSizeMsg;
 	UnidirMessage<BDescVec> bindingUpdateMsg;
+	SyncPoint syncPoint;
 
 	const char *argv0;