Clarify timing concepts in relation to SMIL

[plehegar]

Collate references to SMIL concepts and exceptions. See also https://lists.w3.org/Archives/Public/public-tt/2015Jan/0114.html

Add non-normative diagrams showing the relationship between media times and TTML timeBase="media".

Nigel to add this diagram. Pierre to provide container diagram.

(raised by Nigel Megitt on 2015-04-09)
From tracker issue http://www.w3.org/AudioVideo/TT/tracker/issues/381

[nigelmegitt]

Partially addressed by PR #143, which adds two diagrams and two example fragments of TTML.

[skynavga]

Labeling this for ttml.next and closing without further action. If someone wishes to post a PR that adds clarifying material, then please do so and reopen this issue.

[nigelmegitt]

Further, it would be feasible to create a WG Note describing the relationship between TTML and SMIL timing semantics; this does not need to be done as part of TTML2 necessarily.