Precompute fast cluster fit factors, and store as static const.

nvtt is completely reentrant now. Fixes issue 37.
cleanup interface of cuda compressors.

src/nvtt/squish/fastclusterfit.h

@@ -44,8 +44,6 @@ public:
 	void SetMetric(float r, float g, float b);
 	float GetBestError() const;
 
-	static void DoPrecomputation();
-
 	// Make them public
 	virtual void Compress3( void* block );
 	virtual void Compress4( void* block );