[Bug 791981] hlsdemux: check sequence number as spec defined

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=791981

--- Comment #1 from Jun Xie <jun.xie at samsung.com> ---
#what is current implementation:
1. Find first case of higher/equal sequence number in new playlist
2. From there on we linearly step ahead

#what is current implementation issue:
1. After reloading a Media Playlist, the client SHOULD verify that each
   Media Segment in it has the same URI (and byte range, if specified)
   as the Media Segment with the same Media Sequence Number in the
   previous Media Playlist. 
   -> this is not verified in current code, first case of higher 
   sequence number in new playlist is found as pivot. And old list and new
   list are step ahead. by this way, actually same media sequence number 
   segment is not checked at all.
   e.g.
   *old playlist
    #EXT-X-MEDIA-SEQUENCE:2680
    #EXTINF:8,
    https://priv.example.com/fileSequence2680.ts
    #EXTINF:8,
    https://priv.example.com/fileSequence2681.ts
    #EXTINF:8,\n\
    https://priv.example.com/fileSequence2682.ts
    #EXTINF:8,\n\
    https://priv.example.com/fileSequence2683.ts
   *new playlist
    #EXT-X-MEDIA-SEQUENCE:2682
    #EXTINF:8,
    https://priv.example.com/fileSequence2682.ts
    #EXTINF:8,
    https://priv.example.com/fileSequence2683_not_the_same.ts
    #EXTINF:8,\n\
    https://priv.example.com/fileSequence2684.ts
    #EXTINF:8,\n\
    https://priv.example.com/fileSequence2685.ts

    this will be checked as validate, which shall be invalidate.

2. Media Segments MUST be removed from the Playlist file in the order
   that they appear in the Playlist;
   -> so new Playlist file's 1st sequence shall not be smaller than what in the
older.
   -> so new Playlist file shall not be a subset of the older.

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