display: Force window user time update to happen

`meta_window_set_user_time()` will not update the window's
user time if it timestamp in the argument is before the
currently saved timestamp. However, when trying to work around
problematic timestamps, this is exactly what needs to be done.

So force the update to happen by setting the "is user time set?"
flag to false.

Fixes: 8f3da9f68a ("Use meta_window_set_user_time for setting user time consistently")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3027>

src/core/display.c

@@ -2494,6 +2494,7 @@ meta_display_sanity_check_timestamps (MetaDisplay *display,
               meta_warning ("%s appears to be one of the offending windows "
                             "with a timestamp of %u.  Working around...",
                             window->desc, window->net_wm_user_time);
+              window->net_wm_user_time_set = FALSE;
               meta_window_set_user_time (window, timestamp);
             }