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.