Incorrect order in foreground objects (wrong object visibility)

I am far from proficient with 3D viz and not sure this is a mayavi issue as such (apologies if I bark up the wrong tree), but I observe the following, unexpected behavior:

## Environment:

conda, anaconda 3, installed mayavi using `pip install mayavi vtk pyqt5` rather than conda (after bumping into a similar issue to #652)

```
Debian 4.9.88-1+deb9u1 (2018-05-07) x86_64 GNU/Linux
Name: mayavi
Version: 4.6.0
Name: vtk
Version: 8.1.0
Location: /home/xxxyyy/anaconda3/envs/ELA/lib/python3.6/site-packages
```

## Repro

```py
# example from https://stackoverflow.com/a/10740780
from mayavi import mlab
import numpy as np

# Generate some random hypocenters
x, y, z, mag = np.random.random((4, 500))

# Make a curved well bore...
wellx, welly, wellz = 3 * [np.linspace(0, 1.5, 10)]
wellz = wellz**2

# Plot the hypocenters, colored and scaled by magnitude
mlab.points3d(x, y, z, mag)

# Plot the wellbore
mlab.plot3d(wellx, welly, wellz, tube_radius=0.1)

mlab.show()
```
The colored balls should surround the grey pipe, hiding portions of it, below.
I do get the expected behaviors with a prior version of mayavi on a different OS- Windows (mayavi version TBC, probably 4.5)