This is a fix on top of the previous issue fixed in
c917965b.

This was caught by "libFuzzer" testing.

No reason to not have it.  Makes life easier later.
We (hb-ft, hb-ot-font, etc) can use this API to inject new
parent into a font...

This changes the default load_flags of fonts created using
hb_ft_font_create() from NO_HINTING to DEFAULT.  Hope that doesn't
break too much client code.

Code calling hb_ft_font_set_funcs() is unaffected.

User can define hb_malloc_impl, etc, to name of custom allocator functions
that have the same signature as malloc.

Makes for a better default and avoids nasty inheritance issues.
See mailing list thread "Default hb_font_t scale".

We don't rely on that.  However, whenever hb_font_make_immutable()
is called, it makes its parenting chain immutable.

Part of https://github.com/behdad/harfbuzz/issues/136

Move the empty-buffer check from hb_shape_full() to hb_shape_plan_execute().

Reported by Simon Cozens.

Not functional change (expected!).

Fixes possible invalid read of two bytes.

Reported by Behzad Najjarpour Jabbari, Secunia Research.

As measured / improved by Benson Limketkai.

The BCP-47 registry defines a variant subtag "fonipa" that can be used
in combination with arbitrary other language tags. For example,
"rm-CH-fonipa-sursilv" indicates the Sursilvan dialect of Romansh
as used in Switzerland, transcribed used the International Phonetic
Alphabet.

http://www.iana.org/assignments/language-subtag-registry/language-subtag-registry

Fixes https://github.com/behdad/harfbuzz/issues/103

A while back we marked every API as version:1.0.  We should fix them all
to reflect real version they were introduced.  This is a start.

Patch from Nikolay Sivov.

This can possibly produce more granular clusters.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=541608
and cluster test.

Currently fails.
https://bugzilla.gnome.org/show_bug.cgi?id=541608

Needed for upcoming merge-clusters fix.

Use gtk-doc to build user's manual as well as reference