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

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.

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.