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.

Not quite useful just yet.

Should have been done as part of da975419

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

Makes way for adding arbitrary user_data support.

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

This is the common convention for language binding tools.

Graphite module not updated.
Bump version to 0.3.

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.

So we can apply it to class methods also.  Not sure if that works!

In anticipation for buffer revamp coming.

Not used yet.