function decompositionSubstitutionFormat1(glyphIndex, subtable) {
    var substituteIndex = lookupCoverage(glyphIndex, subtable.coverage);
    if (substituteIndex === -1) { return null; }
    return subtable.sequences[substituteIndex];
}
Map a feature tag to a gsub feature
@param {any} features gsub features
@param {string} scriptTag script tag