Fix bug #24261: Area under Objectives not redrawn on resize

This stops transient dialogs from undrawing themselves under
SDL2. Instead we rely on redrawing.

src/gui/dialogs/transient_message.cpp

@@ -63,6 +63,10 @@ void show_transient_message(CVideo& video,
 	ttransient_message dlg(
 			title, title_use_markup, message, message_use_markup, image);
 
+#if SDL_VERSION_ATLEAST(2, 0, 0)
+	dlg.set_restore(false);
+#endif
+
 	dlg.show(video);
 }