Reduce hourglasses by caching snapshots

[gwhitney]

After #506 is complete, if one does a sequence switcher when Turtle is the active visualizer, there will always by $T-N$ hourglasses showing at first, where $T$ is the number of thumbnails visible and $N$ is the allowed max of WebGL contexts.

Reduce this sometimes by caching snapshots keyed by query, either

(I) in the browser localStorage, or

(II) on the server.

(Some pros/cons: we have much more space, and sharing between clients, on the server; but we do have to implement running frontscope headless on the server to generate the snapshots in that case.)