Support for Sprite

[manymuch]

Thanks for your great work, could you add support for sprite for visulization of pure 2D image, as discussed here:
https://stackoverflow.com/questions/36668836/threejs-displaying-a-2d-image

[kingjin94]

Depending on the application billboards might be the more applicable. In three.js they seem rather straight forward: https://threejs.org/manual/#en/billboards.

MRs and other resources relevant to its implementation:

• On meshcat's js side: add 2d text (as the texture of a plane)  meshcat#43
• On meshcat's python side: add interfacing to javascript side 2d text rendering #32

[kingjin94]

I would propose using billboards (cleaner and better readable). These MRs tackle their integration:

• Add billboard capability meshcat#145
• Add billboard capability #139