Route native close through platform services

src/app.cpp

@@ -27,7 +27,6 @@ bool async_lock_try();
 void async_lock();
 void win32_async_swap();
 void async_unlock();
-void destroy_window();
 void win32_renderdoc_frame_start();
 void win32_renderdoc_frame_end();
 #elif __LINUX__
@@ -273,16 +272,7 @@ bool App::request_close()
         m->m_message->set_text("Do you want to close without saving?");
         m->btn_ok->m_text->set_text("Yes");
         m->btn_ok->on_click = [this](Node*) {
-#ifdef _WIN32
-            destroy_window();
-            //PostQuitMessage(0);
-#elif __OSX__
-            dispatch_async(dispatch_get_main_queue(), ^{
-                [osx_view close];
-            });
-#elif __LINUX__
-            glfwSetWindowShouldClose(glfw_window, GLFW_TRUE);
-#endif
+            request_app_close();
             Canvas::I->m_unsaved = false;
         };
         m->btn_cancel->m_text->set_text("No");