Initial plant representation and generation

[jonas-schievink]

TODO (before this gets merged):

• Recursively generate branches growing out of the trunk
• Get rid of Params, just use &'static Preset in the generator

TODOs after this PR:

• Replace the old plant type with the new one and make everything compile
• Prototype plant renderer (line strip)
• Make real plant rendering work (using a tessellation shader)
• Leaves
• Configurable branch spawn chance
• More tree presets (eg. for bushes, maybe cacti, etc.)
  • Extend the tree generator/representation to make this possible
• Make tree gen more configurable
• Support other plant types if the tree generator can't handle them (the tree generator is extremely flexible, so we should fully utilize it first)

[jonas-schievink]

r? @LukasKalbertodt