Ensure pythreejs axes render is opaque

[chrisjsewell]

Even though the render is set to opaque:

ase-notebook/ase_notebook/backend/threejs.py

Line 457 in 091e10b

ax_renderer = pjs.Renderer(

In the documentation, it appears not to be:
https://ase-notebook.readthedocs.io/en/latest/user_guide/tutorial_intro.html#threejs-3d