Boundary at lower image edge does not appear in 3D

## Overview

An image region that extends to the border of the region is not rendered with a boundary at the image edge.

## Expected behavior

There is a visible border distinction in 3D volume rendering separating image contents from the background outside of the image.

## Observed behavior

No border is drawn at the lower edge of an image whose feature region extends to the lower bounds of the image.

## Minimum Reproducible Example

```py
arr1 = np.zeros([25,25,25], dtype=np.uint8)
arr1[:15,:25,:15] = 1
image1 = itk.image_view_from_array(arr1)

itkwidgets.view(image1)
```

## Platforms

itkwidgets 1.0a33
itk v5.3.0
Python 3.8 / JupyterLab 3.0.9 (CodeOcean)
Google Chrome
Windows 10

## Notes

Behavior may be due to default vtk-js handling of volume rendering at image bounds, uncertain whether there are parameters that could be tweaked to addressed.

cc @PaulHax