This makes a lot of code safer.  We only try modifying the object in one
place, after making sure it's safe to do so.  So, do a const_cast<> in
that one place...

Simplifies hb_in_range() calls as the type can be inferred.
The rest is obsessiveness, I admit.

Avoids USHORT, SHORT, ULONG, LONG clashes with Windows API.

Should fix warnings generated when building with -Wunused-private-field.
Based on patch from Jonathan Kew.

More to come, for APPLY, CLOSURE, etc.

gdb was showing <anonymous enum> instead of useful stuff, so name
all our enums.

Where causing issues for people with MSVC.

Also change the Version type to avoid similar bugs in the future.

Reported by Grigori Goronzy.

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.

get_contour_point now takes glyph id before point_index.

get_kerning now takes a vector to fill-in.

So we can liberally use the simple features of C++ that parts of the
codebase is already using.

Helps with avoiding many "extern C" declarations in source files.

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.