function identity$3(out) {
    out[0] = 1;
    out[1] = 0;
    out[2] = 0;
    out[3] = 0;
    out[4] = 1;
    out[5] = 0;
    out[6] = 0;
    out[7] = 0;
    out[8] = 1;
    return out;
  }
Set a mat3 to the identity matrix

@param {mat3} out the receiving matrix
@returns {mat3} out