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 is not taking account of presentationTimeOffset and in some methods is not taking into account the Period start time.