Such that hmtx accelerator can be used from libharfbuzz-subset.

Fixes https://github.com/harfbuzz/harfbuzz/issues/761
Fixes https://github.com/harfbuzz/harfbuzz/issues/762

Untested, but compiles.

https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern

I had forgotten about those types.

Not functional change (expected!).

This makes a lot of code safer.  We only try modifying the object in one
place, after making sure it's safe to do so.  So, do a const_cast<> in
that one place...

Avoids USHORT, SHORT, ULONG, LONG clashes with Windows API.

Should fix warnings generated when building with -Wunused-private-field.
Based on patch from Jonathan Kew.

More to come, for APPLY, CLOSURE, etc.

Oops!