OBJ file format for export

[cchexcode]

I tested a few things using Forjot to create some shapes.
In order to export / import these I needed the OBJ format. That's what I added as export function here and thought you might be interested in having this native inside of the application.

The export uses a crate I built a few months / years ago (wavefront_rs). OBJ format is implemented. MTL format will follow.

[hannobraun]

Thank you for this pull request, @replicadse! I'm in a bit of a hurry right now, but I'll be able to take a closer look on Monday.

CI is failing though. Looks like the new error variant introduced an error in fj-app.

[cchexcode]

Cool - yeah I saw that this morning. Fixing that error today.

[cchexcode]

@hannobraun got the pipeline fixed now - lmk if you have questions

[hannobraun]

Nice! I'll review it tomorrow.

[hannobraun]

Thank you for this pull request, @replicadse! Looks great!

One observation, if I export the test model as .obj and open the exported file in PrusaSlicer, it warns about open edges:

I'm not sure what that's about. I confirmed that it does not do that for 3MF or STL. For all I know it could be a bug in PrusaSlicer, or your library, or the code you submitted here (although I see nothing suspicious).

In any case, I'm going to merge this, as some .obj support is better than none 😄

If there indeed is a problem, we can address it later.

[cchexcode]

Thanks @hannobraun I will check it out. I also plan to make some qol updates to the wavefront library incl materials support etc.
Will make a PR accordingly and check the edges stuff. Also, I think there's probably a more optimal solution to the export than giving every triangle it's own three points (index list / vertex list?).
Thanks!