circle_2d is broken in ipyvolume scatter viewer

[Carifio24]

The ipyvolume viewer currently exposes four geometry options: box, diamond, sphere, and circle_2d. I assume that circle_2d is supposed to draw a flat circle like what we have in the vispy scatter viewer, but it doesn't currently work:

https://github.com/user-attachments/assets/2c045001-7c3a-4e17-a4fa-ea50547bd25a

I believe the cause is https://github.com/widgetti/ipyvolume/issues/317.

I tried fixing the bad reference mentioned in https://github.com/widgetti/ipyvolume/issues/317#issuecomment-665757725 (the variable name has changed, but the issue is essentially the same). While that fixed the issue with the shader compilation, the circles still don't show, so there must be more to the problem.