Updating Airstrike - destroys guard tower/turret

Airstrike now destroys basic turrets/GT.
Should not destroy most buildings, though. (Apparently some people were complaining about it being usde for "sniping" buildings too much).
To make damage vs. buildings relatively consistent regardless of how many tiles they occupy, napalm has far less spread now, but much more damage.

Vulcan cannon does more damage vs. heavy armor, but does not damage buildings much. This is because damage it did to buildings varied too much based on building footprint.