function identity$4(out) {
    out[0] = 1;
    out[1] = 0;
    out[2] = 0;
    out[3] = 1;
    out[4] = 0;
    out[5] = 0;
    return out;
  }
Set a mat2d to the identity matrix

@param {mat2d} out the receiving matrix
@returns {mat2d} out