Exporter: Curve support

[dvicini]

It would be great if the exporter could export curves in a format complying with the recently added curve support in Mitsuba 3.

[njroussel]

When writing the curve shapes I spent some time looking into different file formats. Ideally we would have wanted to accept a input file which came straight out of Blender.

In short, Blender currently doesn't have a "reasonable" export mechanism for curves. The existing exporters either triangulate the curve or it must be part of a larger scene representation like alembic which is overkill for us.

If anyone wants to try implementing this, it is still possible! It requires accessing the particle system information directly from the Blender Python API. We actually have a script for this, I'll happily give pointers.