SVG is unused. Just keeping it here.

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_font_set_ptem()
hb_font_get_ptem()

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

We're not going to define public macros for tags for tables.
There's little use to those and they are implementation detail.

The 'avar' table does not allow random access to axis maps,
so change API to avoid quadratic-time implementation.

Removed -hb_ot_var_normalize_axis_value(), added
+hb_ot_var_normalize_variations() and
+hb_ot_var_normalize_coords() instead.

With this, I believe MATH table API is not ready to be merged.

Now it is just reporting one error about “503 undocumented or incomplete
symbols”, that will be the easy part I guess.

Some of it (create, reference, destroy) are adapted from Cairo docs.

Otherwise, almost all API functions are not extracted.

Use chapter ids instead of numbers, so that we can reorder them,
introduce new ones etc. without the numbers becoming out of date.