[MPEG-OTSPEC] TTC "Offsets2" and slot numbers

[Skef Iterum]

I had a thought about the TTC portion of the combined Google proposal 
after the meeting.

I don't have much experience with the client-side of TTCs. I believe 
that in some cases one can pick out a font inside based on an 
identifying string. For example, the CSS font specification indicates 
that a font is picked out of its container by way of its PostScript 
name. However, I believe I've also seen contexts in which a font is 
picked out by slot number.

In any context where slot number is a valid TTC interface, the second, 
overriding offset can potentially make a slot number ambiguous: On one 
client that understands the new specification you get one list, but on a 
different client that only understands the current spec you get a 
different list.

This is not a problem as long as the lists are kept in sync, but unless 
slot numbers are irrelevant -- or we want to push implementations 
towards their being irrelevant going forward -- I think we should add 
some language to the spec indicating that when both offsets are in use 
their entires should be kept in sync, and perhaps even that validators 
can (and should?) flag fonts in which key entries in the name tables of 
the same slot differ between "Offsets" and "Offsets2".

Skef