#15 3D preview: flat heightfields doesn't show up

[Ralf Lehmann]

This is hardly a bug to me, but might
confuse "inexperienced" people...

Whenever a result of a noise engine, function, or
combiner is a plain flat heighfield (0..0, 1..1, whatever),
there's currently simply nothing to look at in the 3D
preview window.

A simple example would be the constant noise engine...