Add option to recalculate blue channel for prenormalized bc5 normals

[Nominom]

B = sqrt(1 - (sqrt(R) + sqrt(G)))

[ptasev]

Is this the right equation? I thought it would be $R^2 + G^2 + B^2 = 1^2$, which turns into $B = \sqrt{1 - R^2 - G^2}$.

[Nominom]

It should indeed be (R² + G²) instead of (sqrt(R) + sqrt(G)). 🦊 Thanks for double checking!