Fix the zoomToFitNodes offsets calculation

[GaryPan-1515]

Checklist

• [x] The code has been run through pretty yarn run pretty
• [x] The tests pass on CircleCI
• [x] You have referenced the issue(s) or other PR(s) this fixes/relates-to
• [x] The PR Template has been filled out (see below)
• [x] Had a beer/coffee because you are awesome

What?

The offset calculation method in the DiagramEngine.zoomToFitNodes has problems. This PR fixes the issue

Before

After

Why?

How?

The nodes bounding box's coordinates were not scaled by the zoom factor causing this bug

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[GaryPan-1515]

@dylanvorster Would you take a look at this fix? Thanks!

[dalex-am]

@dylanvorster Would you take a look at this fix? Thanks!

I fixed zoomToFit method in my project and got same results with one difference: in the end of offsets I got margin * zoomFactor instead of just margin