vo: hide opengl-cb in --vo=help

Avoids stupid questions.

(cherry picked from commit c7790b4526)

video/out/vo.c

@@ -166,7 +166,7 @@ static bool get_desc(struct m_obj_desc *dst, int index)
         .priv_size = vo->priv_size,
         .priv_defaults = vo->priv_defaults,
         .options = vo->options,
-        .hidden = vo->encode,
+        .hidden = vo->encode || !strcmp(vo->name, "opengl-cb"),
         .p = vo,
     };
     return true;