Fixes https://github.com/behdad/harfbuzz/issues/474

SFNSText has kerx table which apparently is applied.

Should fix https://github.com/behdad/harfbuzz/issues/360

Attempt at fixing #360

Towards implementing optical sizing.  Untested; won't compile.

https://github.com/behdad/harfbuzz/issues/360

https://github.com/behdad/harfbuzz/issues/360

Turns out I don't need this to resolve CoreText optical sizing
issue after all.

https://github.com/behdad/harfbuzz/issues/360

New API:
hb_font_set_ptem()
hb_font_get_ptem()

Needed for hb-coretext optical sizing:
https://github.com/behdad/harfbuzz/issues/360

https://github.com/behdad/harfbuzz/pull/555

* hb-buffer.h: Mark hb_buffer_diff() for export

This will fix the tools builds on Visual Studio, as the symbol is used
by the tools.

* build: Adapt NMake Makefiles for GLib 2.53.4 or later

glib-mkenums was ported from a PERL script to a Python script, so we
need to update how we generate the enum sources for HarfBuzz-GObject in
the NMake builds.  Let this be known in the build documentation for MSVC
builds.

One of the problems with the underlying cmd.exe that the NMake Makefiles
run on is that shebang lines are not recognized, so we need to to test
run the script with Python and see whether it succeeded by outputing a
source file that is larger than 0 in file size (since running the PERL
version of the script will clearly fail and cause an empty file to be
created).

If it succeeds, we then run a small Python utility script that makes the
necessary string replacements, and we are done.  If that fails, then we
run the glib-mkenums script with PERL, and do the replacements with the
PERL one-liners as we did before.

We need to make replace.py use latin-1 encoding when using Python 3.x to
cope with the copyright sign that is in the generated enum sources.

Fixes https://github.com/behdad/harfbuzz/issues/477

This O(N^2) was introduced in 8b2c94c4.
Make it O(N).

Fixes https://github.com/behdad/harfbuzz/issues/509

We are going to implement Unicode Arabic Mark Ordering Algorithm:

  http://www.unicode.org/reports/tr53/tr53-1.pdf

which will reorder marks out of their sorted ccc order.  Adjust
normalizer to stop combining as soon as dangerous ordering is
detected.

This reverts commit 5a330575.

Proper fix coming soon.

Fixes https://github.com/behdad/harfbuzz/issues/503

Fixes https://github.com/behdad/harfbuzz/issues/516

This should affect mark attachment in vertical text.  I have no font to test,
but this sounds the right order.

Noticed while debugging https://github.com/behdad/harfbuzz/issues/532

Fixes https://github.com/behdad/harfbuzz/issues/552

Fixes https://github.com/behdad/harfbuzz/issues/528

Related to https://github.com/behdad/harfbuzz/issues/528

Apparently a base glyph can also become an attached component of a
ligature if the ligature-forming lookup used IgnoreBase.  This was
being confused with a non-first component of a MultipleSubst and
hence not matched for mark-attachment.  Tweak test to fix.

Fixes https://github.com/behdad/harfbuzz/issues/543

Fixes https://github.com/behdad/harfbuzz/issues/551