3MF export is partially broken because of triangle orientation

[hannobraun]

Here's a 3D-printed version of the star model:

The star-shaped hole is missing. This is most likely related to validation errors of the exported 3MF file:

This is not surprising, as the kernel doesn't really have any concept of surface orientation. What wasn't clear so far, is that this affects the validity of exported 3MF files.

#339 is related, as both bugs have the same root cause. Another related (and now closed) issue is #173.

We should prevent such issues outright, by adding a 3MF validation step to the CI build (see #54).

Thanks to @dcompoze for reporting this on Matrix!

[hannobraun]

An additional thought: As part of addressing this, it would probably be a good idea to revert #289, to make orientation bugs more visible.

[hannobraun]

I've started looking into this.