function SBIXGlyph() {
    return _TTFGlyph.apply(this, arguments) || this;
  }
This class is transforms TrueType glyphs according to the data from
the Apple Advanced Typography variation tables (fvar, gvar, and avar).
These tables allow infinite adjustments to glyph weight, width, slant,
and optical size without the designer needing to specify every exact style.

Apple's documentation for these tables is not great, so thanks to the
Freetype project for figuring much of this out.

@private