I don't think I broke anything.  Fuzzers will let me know..

I have no way to authoritatively know, but looks like test font only has one
face.  So, adjust test expectation instead.

Fixes https://github.com/harfbuzz/harfbuzz/pull/1085

Technically, static const needs an out-of-class definition.  Eg:

  CXXLD    libharfbuzz-subset.la
Undefined symbols for architecture x86_64:
  "OT::FeatureVariationRecord::min_size", referenced from:
      bool OT::GSUBGPOS::subset<OT::PosLookup>(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
      bool OT::GSUBGPOS::subset<OT::SubstLookup>(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
  "OT::Record<OT::LangSys>::min_size", referenced from:
      OT::Script::subset(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
  "OT::IntType<unsigned short, 2u>::min_size", referenced from:
      OT::Script::subset(hb_subset_context_t*) constin libharfbuzz_subset_la-hb-subset.o
      OT::RecordListOf<OT::Feature>::subset(hb_subset_context_t*) const  in libharfbuzz_subset_la-hb-subset.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status
make[4]: *** [libharfbuzz-subset.la] Error 1
make[3]: *** [all-recursive] Error 1
make[2]: *** [all] Error 2
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
Exited with code 2

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

The get_table imple was wrong, as table offsets in a dfont are
relative to the resource.  We were treating them as relative to
the big blob itself.  To be fixed.

Part of https://github.com/harfbuzz/harfbuzz/pull/1085

Test still fails, because we do not serialize tables in the same
order that fonttools subsetter does.

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

New API:
hb_face_get_table_tags()

Fixes https://github.com/behdad/harfbuzz/issues/560

Not functional change (expected!).

This makes a lot of code safer.  We only try modifying the object in one
place, after making sure it's safe to do so.  So, do a const_cast<> in
that one place...