Set up an architecture to minimize the use of the baked texture debug setting.
Instead concentrating on setting a per-region flag at the region handshake point.
This should be processed once the new regions are using the updated handshake.
The debug setting is being used in this one location as a placeholder.

Builds, but not fully tested/commented yet, passing this work off to Vir.

I recently tried to wade through llhandle.h and got somewhat perplexed. Armed
with an explanation from Richard, I've added notes to the file to try to make
it a bit less mysterious.

Leaving llhandle.h in llui restricts the set of viewer project directories
which could potentially use it, and there's nothing whatsoever UI-specific
about it.