Preliminary impl of fire spreading.

Fire spreads to one tile adjacet to each burning tile per turn, in order
to fire resistance, once the burning tile expends half of its fuel.
Burn turns counter counts down to 1.

It used to count down to 0, then turn the tile to ash. Now it counts
down to 1, because that's more intuitive.
Default fire resistance of tiles is 1.

A tile with no fire stats specified is thus inflammable.