owner_id pointer.
Also rename the callback. That way, we can keep moving toward a more
generic handling of those embedded IDs (think e.g. about copy code).
This new option converts the stroke to outline perimeter as soon as is drawn.
If no alternative material is set, the actual material is used.
The algorithm is similar to the new operator in D15664
Reviewed By: pepeland
Differential Revision: https://developer.blender.org/D15738