qtdemux: Don't skip track parsing regardless of duration

All the boxes in the 'trak' container should be parsed regardless of whether its duration is shorter than the duration of the longest track.

For example, qtdemux can not parse a video track from sample mp4 created with the following command: gst-launch-1.0 videotestsrc num-buffers=300 ! x264enc ! h264parse ! qtmux name=mux ! queue ! filesink location=sample.mp4 audiotestsrc num-buffers=3000 ! fdkaacenc ! aacparse ! mux.