Import 3D Models from .3mf files

[rubenwe]

Based on @mcenal's suggestion in #5

3mf import is not as trivial as other formats. It can include multiple models and stores transformations for those models (i.e. how they are scaled, rotated etc.).

This needs some design work to decide how those multi part models should be picked up:

• single file = single model
• single file = multiple models
• single file = composite model (Allow designation of groups of models as members of a composition #57)

[thetrebor]

There's a possible easier solution. The 3mf spec also says there's a thumbnail and preview that can be packaged with it. Rather than having the render the model (which may be nontrivial) you can support importing the pre-rended packaged thumbnails in the bundle.

[rubenwe]

@thetrebor - that is indeed one way of getting it into the library faster. The graphics for the library need to be (2^n x 2^n) - but that could, of course, be faked by either scaling the longer side to the nearest possible resolution and centering on a blank texture - or by stamping out based on the shorter side.

The 3D viewer part needs the models non the less. So sooner or later support for the models would need to be there.