lavf/subtitles: add ff_subtitles_queue_seek().

This function is almost identical to lavf/assdec:read_seek2(). It
performs a generic seek for text subtitles demuxers for the new seeking
API.

The only difference with assdec:read_seek2 is the ts_diff being
unsigned to avoid overflows.

The seek callback in the ASS demuxer will be removed when it is
redesigned to use FFDemuxSubtitlesQueue.
