This is tricky to explain, so rather than explain what was wrong, I'll describe
how it works now.
Whenever a ScopedTempoMapOwner::in() call occurs, we check the current thread-local
tempo map ptr. If it is not owned by us (and we have a local tempo map that we
want to use), set it so that it is.
We continue to fetch() the global tempo map ptr back into the thread-local ptr
when the local scope depth drops to zero.
1114a05f1f