Fixes https://github.com/behdad/harfbuzz/pull/23

Also fix hb_assert_constant_t.

Simplifies compatibility issues by centralizing all boilerplate code
in hb-private.hh.

We might want to tweak this some more.  For now, never skipping over
it is better behavior than always skipping.

Part of https://github.com/behdad/harfbuzz/issues/554

CoreText "point"s are not typographic points, but CSS pixels.  Ie.
they are 96 per inch, not 72 per inch.

Fixes https://github.com/behdad/harfbuzz/issues/435

* Clarified wording of syllable/cluster behavior in Uniscribe in final_reordering; changed one other probable typo.

* Additional syllable/cluster swap in comments for final reordering and for initial-reordering matra decomposition.

Another attempt at fully fixing
https://github.com/behdad/harfbuzz/issues/360

Instead of searching for freetype using pkg-config, use the FindFreetype
feature of CMake. This allows for better integration with other projects
that make use of CMake.

Fixes https://github.com/behdad/harfbuzz/issues/518

Fixes bots.

Link libharfbuzz.so with -lm.  Surprising that we survived without
it so far!

When the font size or variations settings on underlying FT_Face change,
one can call hb_ft_font_changed() and continue using hb_font created using
hb_ft_font_create().

Fixes https://github.com/behdad/harfbuzz/issues/559

New API:
hb_ft_font_changed()

New API:
hb_face_get_table_tags()

Fixes https://github.com/behdad/harfbuzz/issues/560

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