TTML2/TTML1 Backwards compatibility analysis

Hi all,

Following yesterday's call, I started an analysis of the possible backwards
compatibility issues of TTML2 vs TTML1. The results are here:
https://docs.google.com/document/d/1Ri7RBBsbIK9SRxA1KsHRejXbYBuL4CRRrbmEZRbwZpg/edit?usp=sharing

This is my analysis, and it might contain errors, oversights. If it is the
case, feel free to comment on it.

With the current status, it looks to me that there is no real backwards
compatibility issue, in the sense that a TTML2 processor would produce a
result, when processing a TTML1 document, that would be acceptable with
what the TTML1 spec indicates.

HTH,
Cyril