Fix snow-wald issue

[Enjoyment_2]

please, loftar, make snow=forest for trees so trees growing on natural forests and covered with snow overlay won't spread wald on side tiles.
it's almost impossible right now to have cultivated forest biome for tree-growing. it will eventually turn into wald.
that's definitely couldn't be the intended mechanic and so should be fixed.
if you don't like snow=forest solution - think again: all foresters will wait for summer no matter of what in case of growing trees, so "I don't like to speed up snowed trees" won't matter, but currently only "not plant trees at autumn at all" is the only solution to preserve natural biomes. and since we need to wait 15 days until trees wake up from spring, it means that 60% of time is off for natural forestry, kinda not okay, IMO