dashdemux: relate wall clock time to timestamps for live streams

Submitted by Thiago Sousa Santos @thiagossantos

Link to original bug (#755115)

Description

Currently dashdemux uses the availabilityStartTime to match timestamps of a live stream with the real world clock. It seems this is not a requirement. See "http://live.unified-streaming.com/loop/loop.isml/loop.mpd?format=mp4&session_id=50375".

This stream fails to play because it tries to play from the first segment that would correspond to 'now' in the wall clock but the mapping is completely wrong so it never finds such segment.

Is there an official way to match timestamps to real world time? Or should we invent a way to do it? Like get the last segment end as 'now' at the beginning of the stream and use that as a base?

Ideas?