seg fault when screenshot of 3D plot

This file: minimal_example_fail.py:
```Python
import faulthandler
faulthandler.enable()

from mayavi import mlab
mlab.test_plot3d()
mlab.screenshot()

```
fails with the following stack:
```
$ python minimal_example_fail.py
Fatal Python error: Segmentation fault

Current thread 0x00007f2c3107e740 (most recent call first):
File "/home/maximilien.chaumon/anaconda3/envs/mne/lib/python3.6/site-packages/tvtk/tvtk_base.py", line 568 in _wrap_call
File "tvtk_classes/open_gl_render_window.py", line 79 in get_pixel_data
File "/home/maximilien.chaumon/anaconda3/envs/mne/lib/python3.6/site-packages/mayavi/tools/figure.py", line 348 in screenshot
File "minimal_example_fail.py", line 15 in
Segmentation fault (core dumped)
```
To reproduce:

```
$ conda deactivate
$ conda create -n test python=3.6 numpy "pyqt>=5" matplotlib traits pyface traitsui
$ conda activate test
$ pip install vtk
$ pip install mayavi
$ python minimal_example_fail.py
```

@larsoner @agramfort