`UPDATE_SNAPSHOTS=1` updates too many snapshots

[emilk]

Ir seems UPDATE_SNAPSHOTS=1 mistakingly updates all snapshots. Instead, it should only update snapshots that are above the threshold

[Wumpf]

as per rerun-io/rerun#10778 (comment) I believe it updates everything that fails the broken pixel threshhold but not things that fail the per-pixel threshhold

(... personally I'm still very unhappy with this two threshhold setup. A classic mean-squared-error approach is much easier to gauge. But don't want to needlessly change a running system either :/)