Initial work for glaciers.

This code will create growing and thawing snow. The snow will grow
if it's storming and temperatures are low, and snow will melt if
the sun is out or some other light source is near. The algorithm
makes very pleasing rounded mountain tops.

From here on, we need to pile snow higher to produce ice blocks, the
next step needed to create glacier-like effects.
Patch error - duplicate method.
Checkpoint.

This isn't ideal yet - the max stack code isn't yet
entirely right as stacks seem to grow too high. I should
also try to limit snow edges to 1-2 snow layer thickness
max as right now things look a bit unbalanced.
Tuned snow accumulation.

Several math errors here, plus tuning some parameters
to make this look better overall. It's fairly decent now.
Don't stack snow up high on vegetation.
clay: also don't create clay above sealevel.

There's otherwise still clay blocks being made in waterfalls
in river biomes, which looks weird once the waterfall
retreats.
Don't need temperature anymore.
Fix snow melt. Snow is crushable too.
Snow accumulation rewrite.

No longer recursive, and tweaked all the parameters to be
a bit simpler and easier to check. The end result is a lot
better than before, since I broke quite a few aspects as
I was going through this code.

Pretty smooth snowy hilltops, growth is smooth as well.

Thawing is done right at the start which simplifies and
makes things a bit faster.
Mostly more glacier-style fixes.

- found a missing break causing stone never to decay
- a thick snow cover will break down everything down to gravel, but
  not to dirt. Grass will still become dirt. This makes mountains
  that are covered with snow become gravel producers
- under snow degradation goes faster, but displacement stays behind
  a bit as it did before.
- lots of minor adjustments
v8
Smooth out thawing, thaw edges.