chore: simplify viewbox cloning

[philippfromme]

Proposed Changes

Uses structuredClone to clone viewbox and simplifies test.

Previously code like this would get a reference to _cachedViewbox and then modify it.

Follow-up to #931

Checklist

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[nikku]

Ok, so this is where things break (we mutate the source), and we do it internally. The fix could also have been to not mutate the returned result.

I'm fine with the proposed fix though (safe in all cases).