by newgrf files. When this is enabled forests cannot be built below the highest
snow line, and farms can't be built above it. Houses still use the
_opt.snow_line so they are all consistent, so to make them respect the snowline
you may want to use some newhouses features as well.
the newhouses grf specs, so all newhouses grfs will be playable in the game.
Many thanks to everyone who contributed code and ideas, and all the testers
who found things we missed.
peter1138