[ttml2] define named metadata items to support SMPTE-TT, EBU-TT interoperability

See [1][2]. I believe this handles most existing metadata usage, but let me
know if I missed something.

[1]
https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#metadata-value-named-item
[2] https://dvcs.w3.org/hg/ttml/rev/f59fffa8b65a

Received on Wednesday, 26 November 2014 22:00:27 UTC