Some more measurable speedup.  The recent commits' speedups are as follows:

Testing with Roboto, ****when disabling kern and liga****:

Before:

FT --features=-kern,-liga
user↦   0m0.521s

OT --features=-liga,-kern
user↦   0m0.568s

After:

FT --features=-liga,-kern
user↦   0m0.428s

OT --features=-liga,-kern
user↦   0m0.470s

So, 17% speedup.

Note that FT callbacks are faster than OT these days since we added an advance
cache to FT.  I don't think the difference is enough to justify adding a cache
to OT.

When not disabling kern, the thing is three times slower, so the speedups
are three times less impressive...  Still, 5% not bad for a codebase that I
otherwise thought is optimized out.

Note that, because of this and other optimiztions in our main shaper,
disabling kern and liga, the OT shaper is now *faster* than the fallback
shaper.  So, that's my recommendation to clients that need the absolute
fastest...

Even without FT or OT font funcs implementing get_nominal_glyphs(), there's measurable
speedup.

Even has measurable speedup...

How come this one is not generated by clang everything bot?!

../../../test/api/test-multithread.c:37:26: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
 static char *font_path = "fonts/Inconsolata-Regular.abc.ttf";
                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../test/api/test-multithread.c:38:21: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]

Towards separating the common case into its own loop.

Unused as of now.  To be wired up to normalizer, which would remove
overhead and allow hb-ot-font initialization to become a no-op, so
we can enable it by default.

Ensure that we have the variables at the beginning of the block.  These
are the only fixes that we need for building HarfBuzz on older
compilers.

Instead of passing a CFLAG/CXXFLAG to define HB_EXTERN, define it
directly in src/hb.hh as __declspec(dllexport) extern when we are
building HarfBuzz as DLLs on Visual Studio.  Define HB_INTERNAL
as nothing without defining HB_NO_VISIBILITY when building HarfBuzz as
DLLs to avoid linker errors on Visual Studio builds.

Also "install" harfbuzz-subset.dll into $(PREFIX)\bin as the
hb-subset utility will depend on that DLL at runtime, when HarfBuzz is
built as DLLs.  Since it consists of private APIs that are subject to
change, we do not install its headers nor .lib file.

Ouch!

Fixes https://github.com/harfbuzz/harfbuzz/issues/250

Fixes https://github.com/harfbuzz/harfbuzz/issues/1232

"The values in the right class table are stored pre-multiplied by the
number of bytes in a single kerning value, and the values in the left
class table are stored pre-multiplied by the number of bytes in one
row. This eliminates needing to multiply the row and column values
together to determine the location of the kerning value. The array can
be indexed by doing the right- and left-hand class mappings, adding the
class values to the address of the array, and fetching the kerning
value to which the new address points."