demux_lavf: also fix cachec 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).

In fact, this shit looks generally a bit gross, why the fuck is our
wrapper compensating for the seek delay manually? (But I checked that
this is really needed. I guess libavformat is really into making people
do gross, nasty shit.)

demux/demux_lavf.c

@@ -646,6 +646,8 @@ static void handle_new_stream(demuxer_t *demuxer, int i)
 
         export_replaygain(demuxer, sh, st);
 
+        sh->seek_preroll = delay;
+
         break;
     }
     case AVMEDIA_TYPE_VIDEO: {