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: https://github.com/rdeits/meshcat/pull/43
• On meshcat's python side: https://github.com/rdeits/meshcat-python/pull/32

[kingjin94]

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

• https://github.com/rdeits/meshcat/pull/145
• https://github.com/rdeits/meshcat-python/pull/139