We need setters and getters for these two very commonly accessed attributes of grease pencil.
The code is based on the implementation of `CurvesGeometry::positions()` and `CurvesGeometry::positions_for_write()`.
Co-authored-by: Falk David <falk@blender.org>
Pull Request: https://projects.blender.org/blender/blender/pulls/109733