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

Untested.

Apparently Uniscribe allows these, and they are used in some Khmer fonts
shipped with Windows, namely, Daun Penh.

This reverts commit 727135f3.

This is work-in-progress.  Didn't mean to push it out just yet.

Shaves a good 10% off DejaVu Sans with simple Latin text for me.
Now, DejaVu is very ChainContext-intensive, but it's also a very
popular font!

To be used for correct mark attachment to first component of a
MultipleSubst output.  That's what Uniscribe does.

Detected by clang and lots of cmdline options.

The old code was doing nothing.

Still got to find an example font+string that makes this matter, but
need this for fixing synthetic GDEF anyway.

The spec doesn't say contextual matching should be done this way,
and AOTS doesn't do it either.  It was inherited from old HarfBuzz.
Remove it.

This is to incorporate it into GSUB/GPOS processing.

More to come, for APPLY, CLOSURE, etc.

The GSUBGPOS part still missing.

Filling in.

To be used in the Indic shaper later.  Unused for now.

gdb was showing <anonymous enum> instead of useful stuff, so name
all our enums.

I couldn't measure significant performance gains out of this; maybe
about 5% (with one million Malayalam strings).  Still, not bad.
But reminds me that optimizing this codebase without profiling first
is simply not going to work.  Oh well...

Mozilla bug 701637 and 714067 combined.

Patch from Jonathan Kew.