Include backface_culling flag in serialization format for TileDefs

This way flowing liquids actually show the backface when specified to
do so. Without this, TileDefs where by default initialized with
backface_culling = true and never set otherwise.

For backwards compatibility, an old client connected to a new server,
or a new client connected to an old server will behave like before
i.e., backface_culling is always true.

src/nodedef.h

@@ -119,7 +119,7 @@ struct TileDef
 		animation.length = 1.0;
 	}
 
-	void serialize(std::ostream &os) const;
+	void serialize(std::ostream &os, u16 protocol_version) const;
 	void deSerialize(std::istream &is);
 };