"Improve" is a strong word in this case though, I understand.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1224

We use scratch-flags to short-circuit this function.  No need for previous
early loop.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1216

New API:
hb_buffer_set_invisible_codepoint()
hb_buffer_get_invisible_codepoint()

hb-shape / hb-view --invisible-codepoint

Since we have get_*_advanes() API now, the overhead is once per shape,
not once per glyph.

Only cmap is warmed-up at set_funcs() time now.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1230

Ugly, but...
Fixes https://github.com/harfbuzz/harfbuzz/issues/1227

If buffer was enlarged, info was being outdated.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1225

https://github.com/harfbuzz/harfbuzz/commit/a01194aaf4c15160330b4042066263b2c963b658#commitcomment-30772041

"The tag space of tags consisting of four uppercase letters (A-Z) with no punctuation,
spaces, or numbers, is reserved as a vendor space. Font vendors may use such tags to
identify private features."

Now that we added morx support, our OS X bot is running them through CoreText
and failing (with a DoS / infinite loop no less!).  Always run tests through
our own shaper.

One early, before script-specific features, one late, after.  Allows font
developers to detect us and behave differently if needed.

Before this change, autoconf was emitting the following warnings:

```
configure.ac:22: warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
```

This reverts commit f4072e8c.
https://github.com/harfbuzz/harfbuzz/issues/1195

End of experiment, we might get back to it later

Fixes https://github.com/harfbuzz/harfbuzz/issues/1159

Also, don't attach ZWNJ to previous cluster.  Closer to Unicode graphemes.