TTML2 IR Complete

Pending further comment resolution on the IR [1], it is now ready to go, at
least as far as the TTML2 transition request is concerned.

Since it is on the W3C wiki, we can edit it in real time as needed, at
least until the PR is published, at which point we should hold off on
further changes until the AC review finishes.

It would be useful for all implementation reporters to review their
appropriate columns in the 4 different tables, particularly in tables 1 to
3. If you make any change in table 4, please coordinate with me so I can
make sure the various summing columns stay correct.

[1] https://www.w3.org/wiki/TimedText/TTML2ImplementationReport