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 would make it faster too).
• Move and rename the function so it's clear that it's meant to be used for primitives only.