vo_sdl: put on do not use stamp

It seems some users try to use it (!). This VO was always an experiment,
and intended for low power devices. Whether this experiment succeeded or
not, it's a rather obscure VO. Recently I've seen a regrettable user,
who seemed to use this only because mpv was built without x11 support
(!). Add this warning, like other fallback VOs have it. (The message was
copied from vo_x11.)

video/out/vo_sdl.c

@@ -809,6 +809,9 @@ static int preinit(struct vo *vo)
     if (vc->wakeup_event == (Uint32)-1)
         MP_ERR(vo, "SDL_RegisterEvents() failed.\n");
 
+    MP_WARN(vo, "Warning: this legacy VO has bad performance. Consider fixing "
+                "your graphics drivers, or not forcing the sdl VO.\n");
+
     return 0;
 }