Support for Track Reference Atoms

I recently learned about the track reference atoms. These are really useful for 
describing the relationship between tracks within an alternate group, and are 
used by Apple TV (and probably other players) to determine the correct tracks 
to use within an alternate group. For example:

'forc' - Marks a subtitle track as "forced"
'fall' - Marks a track as a fallback or another track. For example an AC-3 
audio track could mark an AAC audio track as a fallback.
'folw' - Marks a "following" track. For example, you have two subtitle tracks, 
one forced and one full track. In an audio track, this indicates the primary 
subtitle track, and in a subtitle track, this indicates the alternate subtitle 
track.

There's others that could also be useful, but these are the most important.

What version of the product are you using? On what operating system?

Version 2.0

Please provide any additional information below.

Gory details about these atoms can be found at: 
https://developer.apple.com/library/mac/documentation/QuickTime/QTFF/QTFFChap2/q
tff2.html

Original issue reported on code.google.com by raftisa...@gmail.com on 23 Jan 2015 at 11:31

larod / mp4v2

Support for Track Reference Atoms #175