demux_lavf: let libavformat open HLS streams directly

Fixes opening some streams.

This means the HLS playlist will be opened twice, but that's not much of
a problem, considering it's pretty small, and HLS will make many other
http accesses anyway.

demux/demux_lavf.c

@@ -707,7 +707,8 @@ static int demux_open_lavf(demuxer_t *demuxer, enum demux_check check)
     }
 
     if ((priv->avif->flags & AVFMT_NOFILE) ||
-        demuxer->stream->type == STREAMTYPE_AVDEVICE)
+        demuxer->stream->type == STREAMTYPE_AVDEVICE ||
+        matches_avinputformat_name(priv, "hls"))
     {
         // This might be incorrect.
         demuxer->seekable = true;