Fixes https://github.com/harfbuzz/harfbuzz/issues/1241

Part of https://github.com/harfbuzz/harfbuzz/issues/1241

New API:
+hb_ot_var_axis_flags_t
+hb_ot_var_axis_get_flags

Unfortunate that we don't have room in hb_ot_var_axis_t to expose flags :(.

Was causing more trouble than it solved.  We use unsigned for indexing,
and it's not helpful to allow that wrapping to negative integers on
32bit machines.  The only way we could work around it would have been
by accepting int64_t arg, but that's overkill.

Ignore the MSVC 2008 build issue.  We don't support that compiler.

Was causing bad substitutions in mort table because of WordOffsetToIndex()
producing negative numbers that were cast to unsigned int and returned as
large numbers (which was desirable, so they would be rejected), but then
they were cast to int when passed to this operator and acting as small
negative integers, which was bad...

Detect overflow.  Ouch, however, now I see this still fails on 32-bit.
Guess I'm going to revert an earlier change.

Ouch!

Ouch!

Follow-up to cf203af8

Fixes https://github.com/harfbuzz/harfbuzz/issues/1343

That's what Apple does, and it wouldn't degrade our OpenType performance.

Part of https://github.com/harfbuzz/harfbuzz/issues/1348

Fixes https://github.com/harfbuzz/harfbuzz/issues/1348

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11351

Fixes https://github.com/harfbuzz/harfbuzz/pull/1388

Fixes https://github.com/harfbuzz/harfbuzz/issues/1248#issuecomment-438689499

Add test for https://github.com/harfbuzz/harfbuzz/issues/1379