== Need parameter attribute to express resolved storage aspect ratio
in ISD. ==
Need to pass through to ISD (on isd:sequence or isd:isd) a parameter
expressing the resolved storage aspect ratio (as extent) of root
container region. For example, this parameter may be defined as
ttp:storageAspectRatio but prohibited from appearing in TTML2 source
content; rather, only permitted in ISD content.