dashdemux: include both Period start and presentationTimeOffset in segment start

dashdemux: include both Period start and presentationTimeOffset in segment start

The start of each segment is relative to the Period start, minus
the presentation time offset.

As specified in section 5.3.9.6 of the MPEG DASH specification:
    The value of the @t attribute minus the value of the
    @presentationTimeOffset specifies the MPD start time of
    the first Segment in the series.

dashdemux was not taking account of presentationTimeOffset and in
some methods was not taking into account the Period start time.
This commit modifies the segment->start value to always be
relative to the MPD start time (zero for VOD,
availabilityStartTime for live streams). This makes all uses of
the segment list consistent.

Fixes #841