Cosmetics
Implement variation selectors support

... in CTFont bridge for HarfBuzz.
Implement vertical glyph advance support

... in CTFont bridge for HarfBuzz.
Write absolute filename of Mac fonts to XDV file

This way we can drop loading fonts by name from xdvipdfmx.
Avoid ATSFontGetFileReference() errors

When passing an invalid ATSFontRef to CTFontCreateWithPlatformFont(),
get:
<Error>: ATSFontGetFileReference failed: error -50