pygfx volume example rendering strange

Unsure why, but `examples/basic_volume.py` is rendering strange with pygfx. I've seen it locally, but it also appears on the [website](https://pyapp-kit.github.io/scenex/dev/examples/basic_volume/) now that the examples are being rendered there.

<img width="699" height="782" alt="Image" src="https://github.com/user-attachments/assets/bc2aba3a-7052-438a-9c05-375925069138" />