ui/dbus: fix filtering all update messages

ui/dbus: fix filtering all update messages

Filtering pending messages when a new scanout is given shouldn't discard
pending cursor changes, for example.

Since filtering happens in a different thread, use atomic set/get.

Fixes: fa88b85dea ("ui/dbus: filter out pending messages when scanout")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-ID: <20241008125028.1177932-6-marcandre.lureau@redhat.com>
(cherry picked from commit cf59889781297a5618f1735a5f31402caa806b42)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>