Blender Git Loki

Cleanup: Avoid duplication in line art stroke generation

The BKE_gpencil_stroke_add_points API function worked well for
creating the primitives in the add object menu, but it expected a
specific data format that doesn't make sense in a dynamic context.
As evidence of that we can see the way source data was duplicated
in the line art file just to use this API function.

This commit solves that problem in two ways:
- Clean up the line art function (this should make it faster too).
- Move/rename the function so its intended use is more clear.

Differential Revision: https://developer.blender.org/D11909