orbital: Fix EventLoop::run_app_on_demand

Previously event_loop.exit() would persist and cause all future
run_app_on_demand() calls to immediately exit.

winit-orbital/src/event_loop.rs

@@ -526,6 +526,7 @@ impl EventLoop {
         &mut self,
         mut app: A,
     ) -> Result<(), EventLoopError> {
+        self.window_target.exit.set(false);
         loop {
             match self.pump_app_events(None, &mut app) {
                 PumpStatus::Exit(0) => {