function multiply(out, a, b) {
    var a00 = a[0], a01 = a[1], a02 = a[2], a03 = a[3],
        a10 = a[4], a11 = a[5], a12 = a[6], a13 = a[7],
        a20 = a[8], a21 = a[9], a22 = a[10], a23 = a[11],
        a30 = a[12], a31 = a[13], a32 = a[14], a33 = a[15];

    // Cache only the current line of the second matrix
    var b0  = b[0], b1 = b[1], b2 = b[2], b3 = b[3];  
    out[0] = b0*a00 + b1*a10 + b2*a20 + b3*a30;
    out[1] = b0*a01 + b1*a11 + b2*a21 + b3*a31;
    out[2] = b0*a02 + b1*a12 + b2*a22 + b3*a32;
    out[3] = b0*a03 + b1*a13 + b2*a23 + b3*a33;

    b0 = b[4]; b1 = b[5]; b2 = b[6]; b3 = b[7];
    out[4] = b0*a00 + b1*a10 + b2*a20 + b3*a30;
    out[5] = b0*a01 + b1*a11 + b2*a21 + b3*a31;
    out[6] = b0*a02 + b1*a12 + b2*a22 + b3*a32;
    out[7] = b0*a03 + b1*a13 + b2*a23 + b3*a33;

    b0 = b[8]; b1 = b[9]; b2 = b[10]; b3 = b[11];
    out[8] = b0*a00 + b1*a10 + b2*a20 + b3*a30;
    out[9] = b0*a01 + b1*a11 + b2*a21 + b3*a31;
    out[10] = b0*a02 + b1*a12 + b2*a22 + b3*a32;
    out[11] = b0*a03 + b1*a13 + b2*a23 + b3*a33;

    b0 = b[12]; b1 = b[13]; b2 = b[14]; b3 = b[15];
    out[12] = b0*a00 + b1*a10 + b2*a20 + b3*a30;
    out[13] = b0*a01 + b1*a11 + b2*a21 + b3*a31;
    out[14] = b0*a02 + b1*a12 + b2*a22 + b3*a32;
    out[15] = b0*a03 + b1*a13 + b2*a23 + b3*a33;
    return out;
}
Multiplies two mat4's

@param {mat4} out the receiving matrix
@param {mat4} a the first operand
@param {mat4} b the second operand
@returns {mat4} out