Should remove the vector version at some point...

Why does subset plan not have a hb_set_t of glyphs?

Ouch.

Remove Lookup::subset().

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.

We might reintroduce it later, but for now remove, as it was unused.
Some things that should have been in this object (drop_hints, etc)
are already in hb_subset_input_t.  So, for now, keep everything there.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1145

pkg.m4 is provided by pkg-config, which is a requirement to build, and this copy is ancient.  Delete it to avoid it being used instead of the host copy.

clang -O3 was completely removing _get_gdef(), causing link
failure when needed from another compilation unit.  Surprisingly,
"extern inline" didn't fix it.

Fixes clang bots as well as fuzzer issue.

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

I swear I have no idea why this is needed, but it is...