Design prop::Plant while thinking about generation algorithm

[LukasKalbertodt]

This is probably pretty difficult to get right... so it can be experimented a lot.

The task is to think about a smart way to represent plants (meaning: all possible plants in the game). This representation should be compact, kind of "scalable" (compare to: SVG instead of PNG) and possible to generate.

Currently the type contains a height and a stem_width... this is not enough to represent all plants we want to create... it is only able to represent very ugly plants... On the other hand: if we would save a vertex buffer directly, this wouldn't be very scalable and hard to generate directly (and it would be too OpenGL dependent, of course).

So we need some form of smart representation which allows us to compute an vertex buffer of arbitrary precision and is more or less easy to generate. I was thinking of something like multiple curves which also save a "width" at each control point. This would let us represent the branches of a tree (plant) fairly well.

You should definitely talk to me in person about this. And also don't expect the first design to be final... the type will change over time, adding and removing some data.

[LukasKalbertodt]

Done in #36