function defineDependentProperty(glyph, externalName, internalName) {
    Object.defineProperty(glyph, externalName, {
        get: function() {
            // Request the path property to make sure the path is loaded.
            glyph.path; // jshint ignore:line
            return glyph[internalName];
        },
        set: function(newValue) {
            glyph[internalName] = newValue;
        },
        enumerable: true,
        configurable: true
    });
}
@param  {number} index
@return {opentype.Glyph}