Examples of Per-Profile IMSC Specific Presentation Processing by a MPIP

Since folks have wondered what type of semantic processing may need to
discriminate on which profile applies, I have collected a small set of
examples that apply to presentation processing without any consideration of
verification/validation. At least one implementation (TTPE) implements code
to handle these cases:

   - nested tt:div
      - IMSC-T supports nested div, while IMSC-I does not; an MPIP needs to
      respect nesting for the former, but ignore (or collapse divs) or
abort for
      the latter;
   - tts:displayAlign
      - IMSC-T supports all values, while IMSC-I prohibits its use; so an
      MPIP would need to use displayAlign when processing the former,
but ignore
      or abort for the latter;
   - tts:padding
      - IMSC-T supports padding on region, while IMSC-I does not; an MPIP
      needs to respect the former, and ignore (or abort) the latter
   - tts:writingMode
      - IMSC-T supports all values, while IMSC-I prohibits vertical modes;
      so an an MPIP would need to enable all modes for the former, but
ignore or
      abort if vertical mode is used with the latter;

MPIP = Multiple-Profile IMSC Processor