vo_xv, vo_x11: warn that these VOs should not be used

video/out/vo_x11.c

@@ -407,6 +407,8 @@ static int preinit(struct vo *vo)
         goto error;
 
     p->gc = XCreateGC(x11->display, x11->window, 0, NULL);
+    MP_WARN(vo, "Warning: this legacy VO has bad performance. Consider fixing"
+                "your graphic drivers, or not forcing the x11 VO.\n");
     return 0;
 
 error:

video/out/vo_xv.c

@@ -843,6 +843,9 @@ static int preinit(struct vo *vo)
     ctx->fo = XvListImageFormats(x11->display, ctx->xv_port,
                                  (int *) &ctx->formats);
 
+    MP_WARN(vo, "Warning: this legacy VO has bad quality and performance, "
+                "and will in particular result in blurry OSD and subtitles. "
+                "You should fix your graphic drivers, or not force the xv VO.\n");
     return 0;
 
   error: