TUTools - Add support for BC5 normal maps -- both reading and export

[shadowmage45]

Attempt to use the now-included NVTT unmanaged assembly and C# wrapper for texture read/write handling.

Clean up all texture reading and writing to use a single library (currently there are ~2 in use).

• XNA-DTX utils
• NVDXT.exe

Hmm: https://www.gamedev.net/forums/topic/582176-compressed-texture-formats-dxt1-dxt3-dxt5-etc/ n.z = sqrt(1 - saturate(n.xn.x+n.yn.y));// ton.z = sqrt(1 - saturate(dot(n.xy, n.xy)));

Other options:

• https://github.com/deng0/DirectXTexNet
• https://github.com/simontaylor81/DirectXTex.net/
• Nvidia-Texture-Tools ( https://github.com/castano/nvidia-texture-tools )- C# wrapper around C++ library
• TextureSquish ( https://github.com/vpenades/TextureSquish ) - C# native implementation of LibSquish