cs-screen-x11.c: gdk_flush before retrieving updated monitor and

screen information.  This is done in stage.py when responding to
our CsScreen's signals, but we should do it here before rebuilding
the monitor info for sharing with the stage.

libcscreensaver/cs-screen-x11.c

@@ -378,6 +378,8 @@ on_monitors_changed (GdkScreen *gdk_screen, gpointer user_data)
 
     screen = CS_SCREEN (user_data);
 
+    gdk_flush ();
+
     old_monitor_infos = screen->monitor_infos;
 
     reload_monitor_infos (screen);
@@ -394,6 +396,8 @@ on_screen_changed (GdkScreen *gdk_screen, gpointer user_data)
 
     screen = CS_SCREEN (user_data);
 
+    gdk_flush ();
+
     reload_screen_info (screen);
 
     g_signal_emit (screen, signals[SCREEN_CHANGED], 0);