Move canvas async work into app runtime

src/app_runtime.h

@@ -45,6 +45,7 @@ public:
     void notify_render_worker() noexcept;
     void notify_ui_worker() noexcept;
     void prepared_file_task(std::function<void()> task);
+    void canvas_async_task(std::function<void()> task);
 
     void render_thread_tick(App& app);
     void render_thread_main(App& app, std::stop_token stop_token);
@@ -205,6 +206,8 @@ public:
 private:
     void prepared_file_worker_main(std::stop_token stop_token);
     void prepared_file_worker_stop();
+    void canvas_async_worker_main(std::stop_token stop_token);
+    void canvas_async_worker_stop();
 
     std::deque<std::function<void()>> prepared_file_tasklist_;
     std::mutex prepared_file_task_mutex_;
@@ -212,6 +215,12 @@ private:
     std::jthread prepared_file_worker_;
     bool prepared_file_running_ = true;
 
+    std::deque<std::function<void()>> canvas_async_tasklist_;
+    std::mutex canvas_async_task_mutex_;
+    std::condition_variable canvas_async_cv_;
+    std::jthread canvas_async_worker_;
+    bool canvas_async_running_ = true;
+
     std::deque<AppTask> render_tasklist_;
     std::mutex render_task_mutex_;
     std::condition_variable render_cv_;