Incomplete rendering of triangles using test_mesh_sphere()

I wanted to use Mayavi for doing some simple visualizations of ellipsoids with various shapes and orientations, but the resulting surfaces have triangular holes in them. I then ran the test mesh sphere method and experienced the same problem.

I used virtualenv to create a virtual environment.
```
virtualenv tensorviz -p python3.7
pip install mayavi vtk pyqt5
source tensorviz/bin/activate
```

Then I executed the following in the Python console.
```
from mayavi import mlab
mlab.test_mesh_sphere()
mlab.show()
```

The results are shown below. It seems like every other triangle is not rendered.


I have Ubuntu 16.04 and the following packages in my virtual environment.
```
apptools==4.5.0
configobj==5.0.6
envisage==4.9.2
mayavi==4.7.1
numpy==1.18.2
pyface==6.1.2
Pygments==2.6.1
PyQt5==5.14.2
PyQt5-sip==12.7.2
six==1.14.0
traits==6.0.0
traitsui==6.1.3
vtk==8.1.2
```

Does someone know how to fix this issue?