Add refract

[CJT-Jackton]

I think the function refract is missing. The following code is stolen from here. I is the incident vector, N is the normal vector, and eta is the ratio of indices of refraction.

k = 1.0 - eta * eta * (1.0 - dot(N, I) * dot(N, I));
if (k < 0.0)
    R = floatN(0.0);
else
    R = eta * I - (eta * dot(N, I) + sqrt(k)) * N;

Please add it thank you.

[redorav]

I'll take a look this evening and do some tests to see that it behaves like in fxc and I'll add it. Thanks!

[redorav]

Fixed via ec88eb1