Trim recording loop and retain async worker state

src/app_events.cpp

@@ -89,10 +89,19 @@ private:
     std::jthread worker_;
 };
 
-LegacyPreparedFileWorker& prepared_file_worker()
+struct RetainedPreparedFileWorkerState final {
+    void post(std::function<void()> task)
+    {
+        worker.post(std::move(task));
+    }
+
+    LegacyPreparedFileWorker worker;
+};
+
+RetainedPreparedFileWorkerState& retained_prepared_file_worker_state()
 {
-    static LegacyPreparedFileWorker worker;
-    return worker;
+    static RetainedPreparedFileWorkerState state;
+    return state;
 }
 
 [[nodiscard]] GLint rgba8_internal_format() noexcept
@@ -284,7 +293,7 @@ void App::pick_file_save(const std::string& type, const std::string& default_nam
     LOG("App::pick_file_save %s", target.path.c_str());
     if (target.write_on_background_thread) {
         auto* app = this;
-        prepared_file_worker().post([
+        retained_prepared_file_worker_state().post([
             app,
             writer = std::move(writer),
             callback = std::move(callback),