renamed confusing Stack.size to Stack.count

Factored out CFF1 & CFF2 common subsetting code in hb-subset-cff-common.hh

Renamed cff "private" source/headers without the suffix

Subr-flattened charstrings are temporarily re-encoded in ByteStrBuff during "plan" phase, then copied to hb_serialize_context_t during "write" phase

CSOpSet may callback opcode processing "virtual" functions via CRTP

Numer struct may store a value as fixed optionally in addition to int and float

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...

Used to be ascent+descent.  Whatever. This is more convenient for
upcoming changes.

New API:
+hb_face_collect_unicodes
+hb_face_collect_variation_selectors
+hb_face_collect_variation_unicodes

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

New API:
HB_UNICODE_MAX

And use.