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Parse trex and mehd boxes
Also get duration from mvhd if available, use that in addition to track durations to determine total length of file, clear track header struct so we don't read uninitialized garbage from it, and take composition time offset into account for proper ordering when B frames are used. b/15669839 Change-Id: Ibf3e35b5c7299bac11d0e78a391545fc325dd2d0