core: use STREAM_CTRL instead of accessing stream_dvd internals

Some code in mplayer.c did stuff like accessing (dvd_priv_t *)st->priv.
Do this indirectly by introducing STREAM_CTRL_GET_DVD_INFO. This is
extremely specific to DVD, so it's not worth abstracting this further.

This is a preparation for turning the cache into an actual stream, which
simply wraps the cached stream. There are other streams which are
accessed in the way DVD was, at least TV/radio/DVB. We assume these
can't be used with the cache. The code doesn't look thread-safe or fork
aware.

stream/stream_dvd.c

@@ -747,6 +747,14 @@ static int control(stream_t *stream,int cmd,void* arg)
         }
         case STREAM_CTRL_MANAGES_TIMELINE:
             return STREAM_OK;
+        case STREAM_CTRL_GET_DVD_INFO:
+        {
+            struct stream_dvd_info_req *req = arg;
+            memset(req, 0, sizeof(*req));
+            req->num_subs = dvd_number_of_subs(stream);
+            memcpy(req->palette, d->cur_pgc->palette, sizeof(req->palette));
+            return STREAM_OK;
+        }
     }
     return STREAM_UNSUPPORTED;
 }