function isSpace(i) {
              return (
                (glyphs[i].name && glyphs[i].name === 'space') ||
                (txt.length === glyphs.length && txt[i] === ' ') ||
                (glyphs[i].index && glyphs[i].index === 3)
              );
            }
Returns the set of opentype glyphs for the supplied string.

Note that there is not a strict one-to-one mapping between characters
and glyphs, so the list of returned glyphs can be larger or smaller
 than the length of the given string.

@private
@param  {String} str the string to be converted
@return {Array}     the opentype glyphs