Preserve WebGL contexts by replacing thumbnails with snapshots when done

[gwhitney]

There are two parts to this planned change:

(A) In Thumbnails, run the animation until it gets to its (current) stopping point, then replace the Specimen with just a snapshot of the state it has gotten to, as a png displayed on the canvas. Make sure this frees up the WebGL context of the Specimen, if it had one.

(B) When a Thumbnail is first displayed, if it is a WebGL visualizer, and there are more than N (probably 8) active WebGL contexts, show an hourglass (or loading animation) until a WebGL context frees up, at which point start the usual animation and proceed as in part (A).

[gwhitney]

Resolved in ui2 by #517, closing.