[imsc] Pull Request: Explicitly prohibit tt:image when smpte:backgroundImage is present

palemieux has just submitted a new pull request for https://github.com/w3c/imsc:

== Explicitly prohibit tt:image when smpte:backgroundImage is present ==
Close #430

See https://github.com/w3c/imsc/pull/440

Received on Saturday, 14 July 2018 23:39:01 UTC