demux_lavf: also fix cache seeking with large codec delay

Fixes the same thing as the previous commit did with demux_mkv. I'm not
sure if this is correct or a good idea (well, it works with my sample
file).

There are some shady things in this, but describing them would require
too many expletives.

demux/demux_lavf.c

@@ -658,6 +658,8 @@ static void handle_new_stream(demuxer_t *demuxer, int i)
 
         export_replaygain(demuxer, sh, st);
 
+        sh->seek_preroll = delay;
+
         break;
     }
     case AVMEDIA_TYPE_VIDEO: {