Implement DXT3 alpha

Based on reporting I finally figured out why the Ys 6: Ark of Napishtim textures had wrong alpha: they were using DXT3.  Might be the only game I have that uses DXT3...

I initially got the alpha bits wrong, and it was noticeable, so I'm pretty sure it's right now.  At least all the textures look good.

Also, this probably would do well with pshufb as well, heh.

-[Unknown]