[FEAT] Ensure that zoom is correctly working after fit

[tbouffard]

Is your feature request related to a problem? Please describe. During #836, we have suspected that doing a fit then a zoom could not work if internal zoom factor or scale is not set correctly. In that case, the resulting zoom would behave as if there was no initial fit.

Describe the solution you'd like We need to ensure this by adding visual tests and eventually fix the problem. #836 includes a attempt to add tests, we should decide what kind of tests we do

• fit then zoom in then zoom out: we expect the result of the initial fit
• fit then zoom: we expect a zoom based on the fit scale

The issue could happen for both regular fit and fit to center.