Preset focal point is ignored when choosing view type from GUI

**Environment**
- Python 3.6.9
- Mayavi 4.6.2

**Issue**
The preset `focalpoint` seems to be ignored when choosing the view type (ex. `View along the +Z axis`) in GUI.

To be specific, the `focalpoint` is *first* set in the code:
```Python
mlab.view(
azimuth=180,
elevation=60.0,
focalpoint=(0,0,0),
distance=10.0,
figure=fig,
)
```
*Initially*, the plot is correct; the focal point is set to `(0,0,0)`:


However, once I clicked `View along the +Z axis`, the `focalpoint` seems to be reset to the average of all the data (in this case, point cloud), shifting the whole plot.

The plot after clicking: