Final API changes.

Rename OutputHandler::mipmap method to beginImage.
Add NVTT_VERSION.
Add nvtt::version function.

src/nvtt/tools/compress.cpp

@@ -51,7 +51,7 @@ struct MyOutputHandler : public nvtt::OutputHandler
 		verbose = b;
 	}
 
-	virtual void mipmap(int size, int width, int height, int depth, int face, int miplevel)
+	virtual void beginImage(int size, int width, int height, int depth, int face, int miplevel)
 	{
 		// ignore.
 	}