Edges displayed inside the model when height is 0

[MaxGeldner]

Describe the bug
We noticed a little issue with 3D models with a height of 0 when using Edge Materials. There are diagonal lines inside the model (probably the lines separating the polygons?). We are using the STLLoadPlugin to load the models into the viewer. This issue doesn't seem to be apparent when models have a height >0.

To Reproduce
Steps to reproduce the behavior:

1. Create a Model via the STLLoader Plugin that has a height of 0 (probably this is not unique to the STL Models, but we noticed it with this specific loader!)
2. Apply an EdgeMaterial to the model. We are using this Material config:

new EdgeMaterial(this.viewer.scene, {
    id: 'someId',
    edgeColor: [106, 108, 138],
    edgeAlpha: 1
});

Expected behavior
The model should only have visible edges at the outside edges, not inside the model.

Screenshots
You can also see some correctly renedered models, that have a height >0.

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Chrome
• Version: 106.0.5249.61