Some ports disappear in rendering

[NiklasRentzCAU]

With Smart Zoom enabled, ports (at least the ones from an OSGiViz synthesis) appear for one frame, then disappear from the DOM entirely.

From digging around a little, this is caused by the line const parentAbsoluteScale = element.parent.properties.absoluteScale in the depth-map being computed as undefined, causing subsequent NaN values for the port's x and y values.
This is caused by PR #181 and is related to #184, which should fix problems such as these once and for all.