Improve smashing

I added offset properties to tile so that a custom offset can be defined
for items when they are drawn. This used to default to a Y-offset of
-12. For wall smashing, I set the Y-offset to zero so I can use items
that look like walls and fit them in seamlessly with the tiles.

I also added some tags for "smash_top" and "smash_btm" since some walls
need to be smashed together for the isometric geometry.