matroskademux: Set buffer offsets for output

Submitted by Brendan Long

Link to original bug (#723327)

Description

I'm looking at using the buffer offset as a unique identifier for caption/subtitle cues. This HTML mailing list thread is relevant:

http://lists.w3.org/Archives/Public/public-html/2014Jan/0083.html

Specifically:

Is the UA expected to keep a unique internal id for each in-band
TextTrackCue it creates so that it doesn't reinsert a cue that
was inserted previously, but was modified?

To which the answer appears to be, "yes".

The worst-case scenario for this seems to be WebVTT, where the only way to uniquely identify cues is by where they appear in a file:

00:00:00 --> 00:00:05  
Lorem ipsum  

00:00:00 --> 00:00:05  
Lorem ipsum  

So, I'd like to use GST_BUFFER_OFFSET, but it looks like it never sets set for any caption or subtitle formats. As a proof-of-concept, I'd like to implement this for Matroska/SSA, since it seems to be the best-supported cue format right now. Does that make sense?