subsurface_widget: Clear subsurface list after calls to ::draw()

Otherwise the `draw` call in response to `AboutToWait` causes this list
to grow, and eventually hit errors in `sendmsg` due to too many buffered
file descriptors from creating subsurfaces.

Probably the number of places `UserInterface::draw` are called should be
cleaned up. And it shouldn't be called in `AboutToWiat` like this. Some
improvements could probably be made in upstream iced.

winit/src/lib.rs

@@ -1133,6 +1133,7 @@ async fn run_instance<P>(
                     },
                     cursor,
                 );
+                platform_specific_handler.clear_subsurface_list();
                 draw_span.finish();
 
                 if let user_interface::State::Updated {