DO NOT MERGE - CFF2 - Compact Font Format (version 2) table - update

[josh-hadley]

This is a draft of a proposed update to the CFF2 table specification for public review and comment prior to formal submission to the Open Font Format.

Please note: Adobe appreciates any feedback and commentary submitted here for the purpose of improving this proposal prior to the formal review process. We will review, but are not under any obligation to incorporate nor to give credit or compensation for any feedback supplied. The actual document submitted for formal review may differ from the document in this Pull Request.

[alerque]

For "do not merge" / "wip" PRs, I highly recommend marking the PR as a draft using GitHub's functions (see sidebar in the top section under reviewers). That will block accidental merging.