qtdemux: prefer the tfdt timestamp over the buffer's that is less accurate

The tfdt should be more accurate as the buffer timestamp is provided
by the fragmented format manifest and it might just be an approximation.

gst/isomp4/qtdemux.c

@@ -2967,6 +2967,12 @@ qtdemux_parse_moof (GstQTDemux * qtdemux, const guint8 * buffer, guint length,
       GST_DEBUG_OBJECT (qtdemux, "decode time %" G_GINT64_FORMAT
           " (%" GST_TIME_FORMAT ")", decode_time,
           GST_TIME_ARGS (decode_time_ts));
+
+      /* Discard the fragment buffer timestamp info to avoid using it.
+       * Rely on tfdt instead as it is more accurate than the timetamp
+       * that is fetches from a manifest/playlist and is usually
+       * less accurate. */
+      qtdemux->fragment_start = -1;
     }
 
     if (G_UNLIKELY (!stream)) {