function add$6(out, a, b) {
    out[0] = a[0] + b[0];
    out[1] = a[1] + b[1];
    out[2] = a[2] + b[2];
    out[3] = a[3] + b[3];
    out[4] = a[4] + b[4];
    out[5] = a[5] + b[5];
    out[6] = a[6] + b[6];
    out[7] = a[7] + b[7];
    out[8] = a[8] + b[8];
    return out;
  }
Adds two mat3's

@param {mat3} out the receiving matrix
@param {ReadonlyMat3} a the first operand
@param {ReadonlyMat3} b the second operand
@returns {mat3} out