Always call obs_remove_tick_callback() in destructor

Seems like in some scenarios a screenshot thread would still be active
while the ScreenshotHelper object was deleted

src/utils/screenshot-helper.cpp

@@ -39,9 +39,8 @@ ScreenshotHelper::~ScreenshotHelper()
 		gs_stagesurface_destroy(stagesurf);
 		gs_texrender_destroy(texrender);
 		obs_leave_graphics();
-
-		obs_remove_tick_callback(ScreenshotTick, this);
 	}
+	obs_remove_tick_callback(ScreenshotTick, this);
 	if (_saveThread.joinable()) {
 		_saveThread.join();
 	}

src/utils/screenshot-helper.hpp

@@ -36,7 +36,7 @@ public:
 	std::chrono::high_resolution_clock::time_point time;
 
 private:
-	bool _initDone = false;
+	std::atomic_bool _initDone = false;
 	bool _blocking = false;
 	std::thread _saveThread;
 	bool _saveToFile = false;