We need this to set data on objects safely without worrying that some
other thread unsets it by setting it at the same time.

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.

Need to think more about it.

Now that vertical text works correctly, I'm doing a last round
modification of the font-funcs API to simplify.  Expect no more
changes around here.

Starting to get there, but not without yet another round of changes.

I think I know wheere to go now.

The base for vertical is almost ready now.

I like this API *much* better.  Implementation still incomplete, but
horizontal works.

Design not final yet, and in fact I'm going to change it immediately,
but this is an standalone change for itself.

We need to know whether the glyph exists, so we can fallback to
composing / decomposing.  Assuming that glyph==0 means "doesn't exist"
wouldn't work for applications like Pango that want to use different
"doesn't exist" glyph codes for different characters.  An explicit
return value fixes that.

Makes it clear that kerning is in visual order.

Test passing now.

get_contour_point now takes glyph id before point_index.

get_kerning now takes a vector to fill-in.

Not quite useful just yet.

Mutex (and Windows.h by extension) are fairly isolated now.

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