Replace aabb for grassChunks

[EmiOnGit]

What problem does this solve or what need does it fill?

Currently the bounds of the grass chunk are defined by using the Transform and Aabb.
The Aabb however is rather cryptic for a potential user.
Also, assuming that the height is set as constant, the height of the Aabb box is not really needed

Describe the solution you'd like

Replace the Aabb with a Plane2d struct.
The plane will indicate the field which the grass lives on.
To further indicate the height of the grass, the WarblerHeight::Texture variant will take the max height directly.
In case of WarblerHeight::Uniform the height is already given directly.