MPEG DASH has defined a set of new boxes to specify duration, indexes and
offsets of ISOBMFF fragments.

The Track Fragment Base Media Decode Time (tfdt) Box can in particular be
included inside a traf box to specify the absolute decode time, measured on the
media timeline, of the first sample in decode order in the track fragment.

This information can be used by the isomp4 demux to find out the current position of
an MP4 fragment in the timeline.

This patch adds code to isomp4 to:
- parse the tfdt box
- adjust the time/position member of the new segment sent when playback starts

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=677535