Adjusted textures so that on-wall objects work correctly by placing them
one space in front of the wall they should be on.  This fixes a lot of
issues with lighting and rendering rather than placing them in the same
space as the terrain wall cell itself.

This is also the last commit before a major update to how things are
stored in the world, as the code needs to change to allow multiple
things in each cell.  If anything breaks significantly and irreparably,
revert to here!