New Tree Colors

The forest generation and rendering system now supports trees of varying colors. So I was able to add snow-covered trees as well as autumnal-colored trees. It’s a small change, but it makes the snowy areas look a lot more integrated, and it’s an easy way to add some nice colors in by adding autumnal trees.

While implenting this I also did some major refactoring of the forest rendering system, which was less of a small change but doesn’t change how anything looks :-)