Improve smashing

Added offset properties to tile so that a custom offset could 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.