Ouch.

Remove Lookup::subset().

Add subset() call for GSUBGPOS struct and its dependencies.
Not hooked up anywhere.

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

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

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

../../src/hb-ot-layout-gsubgpos-private.hh:391:18: warning: missed loop optimization, the loop counter may overflow [-Wunsafe-loop-optimizations]

Towards possibly using Null pool for some nil objects.

Fixes https://github.com/harfbuzz/harfbuzz/issues/761
Fixes https://github.com/harfbuzz/harfbuzz/issues/762

Fixes https://github.com/harfbuzz/harfbuzz/issues/712

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.

Not optimized to use sortedness yet.  Also start putting in place infra
to faster reject bad data.

A version of Chandas.ttf found on some Chrome bots has 660kb of GPOS,
mostly junk.  That is causing 48 million of set->add() calls in
collect_glyphs(), which is insane.

In the upcoming commits, I'll be speeding that up by optimizing
add_sorted_array(), while also reducing work by rejecting out-of-sort
arrays quickly and propagate the rejection.

Part of https://bugs.chromium.org/p/chromium/issues/detail?id=794896

This only counts recursions right now.  Good start.

Hopefully...
Fixes https://github.com/behdad/harfbuzz/issues/429

Hopefully fully fixes
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=1856

Fixes https://github.com/behdad/harfbuzz/pull/605

Part of fixing https://github.com/behdad/harfbuzz/pull/605