[SDP-US] update to add #length-real

After reviewing the semantics of #length* features, Sean and I believe it
necessary to update SDP-US to add #length-real as a required feature, since
it is not implied by #length-positive.

I've performed this update in order to create an updated version of the WG
NOTE [1] which should be possible to resolve to publish by simple assent in
the WG.

[1] https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10-sdp-us/Overview.html