[ODE] Flat cylinder geom come to life!

[Chris Ledwith]

> There are at least a couple of good uses for true plane geoms:
>
> 1) As a backstop beneath a TriMesh terrain to catch anything that falls
> through the terrain.  (Unlike a trimesh, nothing can fall through a plane
> because the entire back halfspace is "inside" the plane.)
>
> 2) As a primitive to base other primitive implementations upon (e.g.,
> height
> maps).
>
> I'd like to see planes continue to be supported as a first-class
> primitive.
>
> -- jm
>

Me too. They're also useful for invisible walls and ceiling of a (convex)
world/scene.