Blender Git Loki

GP Add Primitives: De-duplicate code

Most of the logic in gp_primitive_rectangle() and gp_primitive_circle()
was duplicated, and responsible for performing space-conversion stuff
on the 2d/screen-space coordinates.

This commit properly separates out these concerns, de-duplicating
code, and fixing a few other minor issues at the same time
(e.g. superfluous copies, and "edges" not getting initialised properly)