Common Regoin for Access Protection.  Like the NullPool, but writable.

Part of https://github.com/harfbuzz/harfbuzz/issues/1017

For consistency.

[subset] sketch out support for a call that lists all codepoints present in a font. Implement support for it in format 4 cmap sub table.

* [subset] sanitize individual DeviceRecord's as part of hdmx sanitization.

* [subset] Fix out of bounds read with non-two byte align glyphs.

* [subset] Just use size_device_record >= DeviceRecord::min_size.

* [subset] Add TODO.

* [subset] Re-order checks in hdmx sanitize.

Fixes https://github.com/harfbuzz/harfbuzz/issues/865

Test still fails, because we do not serialize tables in the same
order that fonttools subsetter does.

sanitizer.sanitize never return nullptr.