Add vertical flip.

src/nvtt/TexImage.cpp

@@ -1086,3 +1086,14 @@ float TexImage::rootMeanSquaredError_alpha(const TexImage & reference) const
 	return float(sqrt(mse / totalCount));
 }
 
+void TexImage::flipVertically() {
+
+    detach();
+
+	foreach (i, m->imageArray)
+	{
+		if (m->imageArray[i] == NULL) continue;
+
+        m->imageArray[i]->flip();
+	}
+}

src/nvtt/nvtt.h

@@ -447,6 +447,9 @@ namespace nvtt
 		NVTT_API float rootMeanSquaredError_rgb(const TexImage & reference) const;
 		NVTT_API float rootMeanSquaredError_alpha(const TexImage & reference) const;
 
+        // Geometric transforms.
+        NVTT_API void flipVertically();
+
 	private:
 		void detach();