qtdemux: handling zero segment-duration edit list

qtdemux: handling zero segment-duration edit list

    Based on document ISO_IEC_14496-12, edit list box can have
    segment duration as zero. It does not imply that media_start equals to
    media_stop. But, it just indicates a sample which should be presented
    at the first. This patch derives segment duration using media_time
    and duration of file. And set derived duration to segment-duration.

    https://bugzilla.gnome.org/show_bug.cgi?id=760781

Change-Id: I965e79f23970153bcc7c3cd935a29a09cc963246