Make smooth transitions a possibility rather than a recommendation

[palemieux]

Closes #304

[palemieux]

Spec at https://rawgit.com/w3c/ttml1/issue-304-smooth-transition-not-preferred-build/index.html

[skynavga]

I oppose this change, since the preference was explicitly part of a WG resolution in the past to resolve a CR comment. What we can do, however, is add an additional note clarifying that this preference applies for ITU Region 2, and that other regions may have different preferences.

[nigelmegitt]

Looks good to me.

[nigelmegitt]

@skynavga Pointer to the WG resolution/decision here please? I do not think we need to mention ITU regions - it is up to the implementer to understand which regional preferences apply and therefore whether or not to implement this behaviour. Appending to the version of the text in this pull request something like "This behaviour is preferred in some regions." would be adequate.

[palemieux]

is up to the implementer to understand which regional preferences apply and therefore whether or not to implement this behaviour.

Moreover, smooth transitions are not applicable to all transitions and all circumstances.