function multiply$6(out, a, b) {
    var a00 = a[0],
        a01 = a[1],
        a02 = a[2];
    var a10 = a[3],
        a11 = a[4],
        a12 = a[5];
    var a20 = a[6],
        a21 = a[7],
        a22 = a[8];
    var b00 = b[0],
        b01 = b[1],
        b02 = b[2];
    var b10 = b[3],
        b11 = b[4],
        b12 = b[5];
    var b20 = b[6],
        b21 = b[7],
        b22 = b[8];
    out[0] = b00 * a00 + b01 * a10 + b02 * a20;
    out[1] = b00 * a01 + b01 * a11 + b02 * a21;
    out[2] = b00 * a02 + b01 * a12 + b02 * a22;
    out[3] = b10 * a00 + b11 * a10 + b12 * a20;
    out[4] = b10 * a01 + b11 * a11 + b12 * a21;
    out[5] = b10 * a02 + b11 * a12 + b12 * a22;
    out[6] = b20 * a00 + b21 * a10 + b22 * a20;
    out[7] = b20 * a01 + b21 * a11 + b22 * a21;
    out[8] = b20 * a02 + b21 * a12 + b22 * a22;
    return out;
  }
Multiplies two mat3's

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