[3v] "Stable" evaluation

[jcreedcmu]

It would be really nice if we could evaluate in such a way that the printed field isn't minimal for each frame, but minimal for the whole evaluation, so that as we animate frames it doesn't jump around.

Otoh, maybe I could fake this on my end if I kept track of the position information, which is not currently being supplied in --json mode, but could be.

[ixchow]

I thought about this but it's hard to know in advance how much to grow the playfield. (In non-json mode the bounds are also printed each frame.)

[jcreedcmu]

I think I can address it in the front end