Improved visual quality of zebra stripes

[rainman110]

This improves the visual quality of the zebra stripe plot in case of low triangulation resolution.

Description

Before, the stripes were computed based on the normal vector of the vertex, not on the fragment. This lead to visual distortions. Now, we use the normal vector from the fragment shader as offered by the occt api.

How Has This Been Tested?

See screenshots

Screenshots, that help to understand the changes(if applicable):

Before:

After fix:

Checklist:

• [ ] A test for the new functionality was added.
• [ ] All tests run without failure.
• [ ] The new code complies with the TiGL style guide.
• [ ] New classes have been added to the Python interface.
• [ ] API changes were documented properly in tigl.h.

[joergbrech]

Thanks @rainman110! This really looks so much better