Dead Leaves and Graphics Improvements

I've made great progress over the last months.

Here's a list of major recent changes:

- Dead leaves

- Updated all the textures in game.

- Improved leaf and stem shading.

- Tree growth is now programmable in Lua. This means it's easier for me to add really different new plants. It also means that the game is now much more moddable. I might even do an online database of plant species, if I could figure out how to filter out most of the dick shaped plants that inevitably will be uploaded there.