Patch and description from Jonathan Kew:

It turns out that some legacy Thai fonts provide OpenType substitution
features to implement mark positioning, but (incorrectly) put those
features/lookups under the 'latn' script tag instead of using 'thai' (or
possibly 'DFLT'). See
https://bugzilla.mozilla.org/show_bug.cgi?id=719366 for an example and
more detailed description.

Although this is really a font bug, I suggest that we could improve the
rendering of such fonts by looking for the 'latn' as a fallback if
neither the requested script nor "default" is found in
hb_ot_layout_table_choose_script. Suggested patch against harfbuzz
master is attached.

This does _not_ affect the other kind of legacy Thai font, where custom
code to support vendor-specific PUA codepoints would be needed. I'm not
keen to go down that path; IMO, such fonts should be ruthlessly stamped
out in favour of standards-based solutions. :)

JK

Since we already switched to accepting -1 as 'zero-terminated'.

hb_face_get_blob() renamed to hb_face_reference_blob(), returns a
reference now.

hb_face_[sg]et_index() added.

hb_face_set_upem() added.

Where causing issues for people with MSVC.

Buffer var allocation coming into shape

I've messed up a lot of stuff recently, different parts of the
shaping process are stumbling on eachother's toes because
manually tracking what's in which buffer var is hard.  I'm
going to add some internal API to track those such that mistakes
are discovered as soon as they are introduced.

Not used for anything right now.  Will use to get num_glyphs in the future.

Needed to be able to include <Windows.h> from hb-object-private.h.

That correctly reflects the reference ownership transfer happening.

Add variant integers to buffer item types.  More cleanup coming.

Not tested at all.

Reported by Jonathan Kew.  Face table handling needs to be redone
anyway, but fix this for now.

hb_font_set_scale() now sets the value to be used to represent a unit
pixel.  For example, if rendering a 10px font with a 26.6 representation,
you would set scale to (10 << 6).  For 10px in 16.16 you would set it to
(10 << 16).  This space should be the same space that the get_glyph_metrics
and get_kerning callbacks work in.

In anticipation for buffer revamp coming.