Towards disabling ZWJ-skipping in certain GPOS contexts.

Part of https://github.com/flutter/flutter/issues/16886

Ouch.  How did we not hit this bug before...

So it doesn't accidentally match our templates, etc.

Use rand=255 to mean "randomize".

Part of https://github.com/harfbuzz/harfbuzz/pull/803

Fix a bug in CBDT sanitize, and redundant check in avar.

Fix avar sanitize().

Fixes https://github.com/harfbuzz/harfbuzz/issues/1110

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.