Move canvas async work into app runtime

src/app.cpp

@@ -689,22 +689,31 @@ namespace
         if (plan.update_frame_label)
             app.update_rec_frames();
     }
+
+    template <typename CanvasDocument>
+    bool process_recording_worker_iteration(App& app)
+    {
+        std::unique_lock<std::mutex> lock(app.rec_mutex);
+        app.rec_cv.wait(lock);
+
+        const auto iteration = make_recording_worker_iteration_context<CanvasDocument>(app);
+        if (!iteration.plan.continue_running)
+            return false;
+
+        if (iteration.plan.encode_frame && iteration.legacy_canvas && iteration.canvas_document && iteration.encoder)
+            encode_recording_frame(app, iteration.plan, iteration.legacy_canvas, iteration.canvas_document, iteration.encoder);
+        return true;
+    }
 }
 
 void App::rec_loop()
 {
     BT_SetTerminate();
     rec_running = true;
-    while(rec_running)
+    while (rec_running)
     {
-        std::unique_lock<std::mutex> lock(rec_mutex);
-        rec_cv.wait(lock/*, [this] { return !(rec_frames.empty() && rec_running); }*/);
-        const auto iteration = make_recording_worker_iteration_context<CanvasDocument>(*this);
-        if (!iteration.plan.continue_running)
+        if (!process_recording_worker_iteration<CanvasDocument>(*this))
             break;
-
-        if (iteration.plan.encode_frame && iteration.legacy_canvas && iteration.canvas_document && iteration.encoder)
-            encode_recording_frame(*this, iteration.plan, iteration.legacy_canvas, iteration.canvas_document, iteration.encoder);
     }
 }