[Bug 777213] hlsdemux: Re-expose only variant stream with bitrate switching

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=777213

--- Comment #15 from Seungha Yang <seungha.yang at navercorp.com> ---
(In reply to tianyahai from comment #14)
> Have you tested m3u8 file that contains mulit bitrate streams and subtitles,
> and can you switch subtile seccessfully? thank you again, have a nice day^_^

I think current gstreamer is not good enough for hls subtitle track change....
DASH + multi TTML case might have similar issue I guess...

In my opinion, current adaptivedemux structure has problem from subtitle track
change point of view.
First, at adaptivedemux side, streaming progress of subtitle is not fully
synchornized with A/V. (since subtitle data is too small, downloading the
subtitle is too faster than A/V).
Adaptivedemux's track change method is that, it tries to download new stream
from 1) last matching streams time position or 2) depending on query position.
This can cause mismatch between old/new subtitle stream's time position.
Second, the fact that multiqueue element doesn't fully consider synchronization
of sparse stream (subtitle stream).

Since I'm not expert... above comment might be incorrect. It's just my opinion.

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