Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10990

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=10955

Untested.

This reverts commit f62f6e90.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1224

This reverts commit f4072e8c.
https://github.com/harfbuzz/harfbuzz/issues/1195

Was getting stuck not making progress somehow.

text-rendering-tests test MORX-32 shows that for Insertion, an unset mark is treated
as mark set at 0.  This is unlike the Reordering lookup where un unset mark performs
nothing.

Fixes MORX-32.

Makes MORX-29, MORX-30, MORX-31 pass.

We reverse too many times. Can be optimized. But I doubt many fonts
use reverse lookups, so doesn't matter.

Other than not applying user features, this completes morx table
implementation.

Just speculation. Needs tests to determine which is correct behavior.

"Unused private member" warning turned error.  ugh.

...like UnsizedArrayOf<>.

This fixes a class of crasher bugs, mostly with color and AAT tables.  We
cannot use nullable offsets to varsized data that does not declare min_size,
because it's nost safe to use our fixed-size null pool for types that have
their size external.  So, use non_null'able offsets for these.

A further enhancement would be to make use of min_size in Null<> itself.
Will try that after.

Does NOT apply user-selected features.  But at least now enables
correct subtables.

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

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

X-terminated means don't access this, it's not located correctly.
Z-terminated means this is a C array with no bound checking.

Splitted from #950 and #986 IIRC Chromium had a policy about this encouraging it,
not sure about automated way to detect and add them but for now lets have the needed
ones of them.