Adding mfra, tfra and mfro box support?

[tamaina]

I am trying to transcode a video in the browser using mp4box.js.

However, the total playback time is not correct due to the playback environment. Adding mfra/tfra/mfro could make the playback time correct.

Please support the addition of mfra/tfra/mfro.

Related to https://github.com/gpac/mp4box.js/issues/243