sub: don't read packets if video and audio are disabled

Although disabling both video and audio is surely an obscure corner
case, it's allowed, and we don't want the demuxer to skip arbitrary
packets.

Basically, make the heuristic for checking interleaved files affect
external files only.

player/sub.c

@@ -64,7 +64,7 @@ static bool is_interleaved(struct MPContext *mpctx, struct track *track)
             (other->type == STREAM_VIDEO || other->type == STREAM_AUDIO))
             return true;
     }
-    return false;
+    return track->demuxer == mpctx->demuxer;
 }
 
 void reset_subtitles(struct MPContext *mpctx, int order)