This adds two new nodes:
* `Grease Pencil to Curves`: Converts each grease pencil layer into an instance
that contains curves.
* `Curves to Grease Pencil`: Converts top-level curve instances into grease
pencil layers.
This opens up many new opportunities:
* Use grease pencil as input to other procedural systems that don't necessarily
output grease pencil.
* Generate grease pencil from scratch using geometry nodes.
* Temporarily convert grease pencil data to curves to use more powerful features
for curves processing.
Some data on layers are not attributes yet unfortunately, so there is some
special case handling for the `opacity` attribute. This was previously discussed
at the geometry nodes workshop:
https://devtalk.blender.org/t/2024-05-13-geometry-nodes-workshop-notes/34760#grease-pencil-14
Pull Request: https://projects.blender.org/blender/blender/pulls/124279
3c81984178