Qt3D / RHI / OpenGL

Compiling / using Qt 6.3.1 now displays the following when running the code:

```
Qt3D.Renderer.RHI.Backend: Initializing RHI with OpenGL backend
Qt3D.Renderer.RHI.Backend: Buffer has no data to upload
```
The plot display(s) are completely white.  Setting the environment variable `QT3D_RENDERER` to `opengl` fixes the issue.