Fix crash/freeze when zooming out too far in a plot

[emilk]

What

We reach i64::MAX pretty quickly when zooming out on e.g. https://github.com/rerun-io/example-rs-github-stars.

In debug builds this overflow would lead to a crash. In release I think it results in a freeze.

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[abey79]

Early signs of year 2038 :)