Route prepared export policy through platform services

src/app_events.cpp

@@ -181,6 +181,11 @@ void App::pick_file_save(std::vector<std::string> types, std::function<void(std:
     active_platform_services().pick_save_file(std::move(types), std::move(callback));
 }
 
+bool App::uses_prepared_file_writes() const
+{
+    return active_platform_services().uses_prepared_file_writes();
+}
+
 void App::pick_dir(std::function<void(std::string path)> callback)
 {
     redraw = true;