Odd visualisation of some models with the default z factor parameter

[lesquoyb]

Describe the bug
The z factor parameter introduced to solve #3857 break some models by adding an increment that seems way too much even though it's supposed to be only 0.05 by default.

For example on the model stairs.gaml in model library:

@tnguyenh showed me a model where the gap between layers was really extreme. I think the problem might lie in the unit of that offset

Expected behavior
There should be no perceivable difference between layers with the default z_factor value

[AlexisDrogoul]

Please test !