Add subset() call for GSUBGPOS struct and its dependencies.
Not hooked up anywhere.

We're going to (finally) use virtual methods for hb_serialize_context_t
customization, so don't need to carry a Serializer template variable
around...  Simplifies code.

Sorry for the noise, downstream custom builders.  Please adjust.

Was creating object even if wasn't there.

Found and fixed a couple bugs.

Found a couple multithreading issues.  Marked them with "XXX-MT-bug".

https://bugzilla.mozilla.org/show_bug.cgi?id=1476334