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.
Add missing bottom face for flowing liquids.

Draw the bottom face of a flowing liquid node if the node below is not
the same liquid, and not solid.

Change the texture to draw for top and bottom faces to the source
liquid texture when the liquid flows straight down, so the liquid
doesn't appear to flow in some random direction.