.fallback is always the default font for the lang group unless we're a
system font (in which case it's "none").
The only reason we need that is because we need to react to
language changes (which affect the initial font).
Simplify the model a bit doing the language lookup in gfxTextRun (this
should avoid allocating a few nsStyleFont structs too.
Depends on D130732
Differential Revision: https://phabricator.services.mozilla.com/D131312
