DXT Compression implemented

DXT compression has been implemented with Nvidia's Texture Tools library. The current trunk does not have functional C# bindings, so I've fixed them and submitted the patch to their Google Code page. Hopefully they'll quickly integrate them in so we don't have to fork and maintain the NVTT separately.

NVTT is supposed to support Windows, OSX and Linux, but I'm only able to build and test it for Windows. Hopefully we'll be able to use similar if not identical bindings?

We'll also be using NVTT for good mipmap generation, rather than having different implementations across platforms.