macOS: Close windows automatically when exiting (#4154)

This disallows carrying over open windows between calls of `run_app_on_demand`
(which wasn't intended to be supported anyhow).

src/platform_impl/apple/appkit/event_loop.rs

@@ -407,6 +407,22 @@ pub(super) fn stop_app_immediately(app: &NSApplication) {
     });
 }
 
+/// Tell all windows to close.
+///
+/// This will synchronously trigger `WindowEvent::Destroyed` within
+/// `windowWillClose:`, giving the application one last chance to handle
+/// those events. It doesn't matter if the user also ends up closing the
+/// windows in `Window`'s `Drop` impl, once a window has been closed once, it
+/// stays closed.
+///
+/// This ensures that no windows linger on after the event loop has exited,
+/// see <https://github.com/rust-windowing/winit/issues/4135>.
+pub(super) fn notify_windows_of_exit(app: &NSApplication) {
+    for window in app.windows() {
+        window.close();
+    }
+}
+
 /// Catches panics that happen inside `f` and when a panic
 /// happens, stops the `sharedApplication`
 #[inline]