Allow for Terrain.Decoration to be scriptable

[ScriptOn (Genya)]

This setting isn’t able to be set by scripts. I would like for it to be configurable from within scripts because I have different parts of the game that sometimes require and don’t require terrain grass & there’s no way to toggle it without using different terrain material types.

[filiptibell]

Introducing Grass

It is not scriptable for a reason. We expect to remove the Decoration setting with the extension to the whole decoration system and we dont want you to be dependent on it in scripts. This way we are making sure that we are not gonna break games much with its (possible) removal in the future.