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.