Fix msvc build. Fixes issue 171.

Improve sse perf.
Fix rmse computation in nvimgdiff.

src/nvtt/CompressorDX9.cpp

@@ -111,7 +111,7 @@ void FastCompressorDXT5n::compressBlock(ColorBlock & rgba, nvtt::AlphaMode alpha
     QuickCompress::compressDXT5(rgba, block);
 }
 
-#if 1
+#if 0
 void NormalCompressorDXT1::compressBlock(ColorSet & set, nvtt::AlphaMode alphaMode, const nvtt::CompressionOptions::Private & compressionOptions, void * output)
 {
     set.setUniformWeights();