qtmux: set tfdt to first dts

This field should be only the first_dts found on the whole timeline and not relatively (dts - fist_dts). Otherwise the fragments generated will always have 0 in tfdt as dts is always first_qt_dts.

This change allows to use mp4mux as a muxer for dashsink and creates proper fragments

$ gst-launch-1.0 videotestsrc num-buffers=500 ! timeoverlay ! openh264enc ! h264parse ! dashsink mpd-root-path=/tmp mpd-filename=experiment_dash.mpd muxer=mp4 target-duration=3

$ gst-play-1.0 /tmp/video_0_1.mp4