Powder Snow - Refinements

A few improvements for Powder Snow would make the game slighty better.

1. From now on, Stray, Frostbite, and Goat can walk on Powder Snow.

2. Powder Snow should be found more often in biomes: Snowy beach/Snowy plains/Snowy slopes/Snowy taiga, so that the winter-based biomes would become a bit more unpredictable and dangerous.

3. New mechanics: Powder Snow melts during the rain.