Fix defense mode

Defense mode's init_mtypes was referencing g->mtypes which is no longer being populated. Changed that over to accessing the MonsterGenerator's information and it now technically works.

There are other bugs related to defense mode that are not within the scope of this branch, namely in how it places monsters. It spams messages from game::update_zombie_pos and game::add_zombie

Then there are interface bugs for window offset which was not changed back when we moved to larger allowed screen sizes for the caravan and the call to setting defense mode parameters.