mesh example renders inverted normals

I'm using the following conda setup:
```
onda list 'anaconda|mayavi|vtk'
WARNING: The conda.compat module is deprecated and will be removed in a future release.
# packages in environment at /home/ondrej/anaconda3:
#
# Name Version Build Channel
anaconda 2019.03 py37_0
anaconda-client 1.7.2 py37_0
anaconda-navigator 1.9.6 py37_0
anaconda-project 0.8.2 py37_0
mayavi 4.6.2 py37h4c70da7_4 conda-forge
vtk 8.2.0 py37haa4764d_200
```

And with the [basic mesh example](https://docs.enthought.com/mayavi/mayavi/mlab.html#id5) I get the following rendering:

It seems to me like there is some strange normal inversion going one. I've tried flipping normals, cull back and front-faces, nothing helped.
It seems to me like the surface has normals on the wrong (or only one) side.

I also ran
```
nosetests mayavi.tests
```
which reported only 7 tests skipped, all the others passed.

Please lets me know how I can debug this. It feels like this might be a simple thing to fix, but I just don't know where.