Why does subset plan not have a hb_set_t of glyphs?

Ouch.

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.

There might be a better way to do this, but I couldn't find...

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

Fixes https://github.com/harfbuzz/harfbuzz/issues/1107

Fixes https://oss-fuzz.com/v2/testcase-detail/5216838347653120
which is a stack overflow, not by way of infinite recursion, just
being deep.  That's disallowed anyway, so catch it as it happens,
not afterwards.

Such a headache that Windows defines UINT8, ...; Just prefix it.

The three "XXXXX"'s should be switched to false.  Doing that separately for ease
of bisecting...

Particularly hazardous if the second layer mixes forward and backward lookups.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4336