[theora] Indexing Ogg files for faster seeking

[ogg.k.ogg.k at googlemail.com]

> Having it as a separate stream does allow adding of additional index
> tracks using existing tools. It makes it relatively easy for a server
> to generate the index track on the fly, add it to an existing ogg
> file, and send that file at a clients request for example.

Skeleton tracks would have to be recreated anyway, whenever an Ogg
stream is modified (eg, tracks added/removed, or chopped).
So adding an index track would presumably require rewriting the Skeleton
track anyway (to give it a fisbone for that track). Unless it's to be considered
metadata rather than an actual track, same as Skeleton, which doesn't
have a fisbone about itself.
OTOH, it's true that a client could say 'I need an index if you have one',
in the same way it could say 'I want Spanish audio if you have it' to a muxing
server.