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

Found and fixed a couple bugs.

Found a couple multithreading issues.  Marked them with "XXX-MT-bug".

Most unused.

Misc. table loading now properly automated.

Also, uncomment code again, just "if (0)" it out, so it doesn't get stale again.

We should hang those off somewhere else. For now, the unused ones can go.

For consistency.

Previously we made CGJ unskippable.  Now, if CGJ did NOT prevent
any reordering, allow skipping over it.  To make this work we
had to make changes to the Arabic mark reordering algorithm
implementation to renumber moved MCM marks.  See comments.

Fixes https://github.com/harfbuzz/harfbuzz/issues/554

We might want to tweak this some more.  For now, never skipping over
it is better behavior than always skipping.

Part of https://github.com/behdad/harfbuzz/issues/554

Makes consumption easier.

Hide them like Mongolian Free Variation Selectors instead.

Fixes https://github.com/behdad/harfbuzz/issues/463

Supports enumerating variation axes, normalizing values, etc.