Rect points

Allows getting "the effective points" of graphic rectangles, round rectangles, and regular polygons.
This allows for some things that are difficult at present: setting the points of a freeform polygon to one of several regular polygon objects; moving an image around the points of a regular polygon, etc.
Quite right - I had F_REGULAR in there and then with sudden loss of clarity deleted it. I'll put it back forthwith. I don't think setting the points of rectangles, rounded rectangles, or regular polygons is a particularly good idea - there's too much validation involved to ensure that the angles and point placements are correct - that's why I enforce the "effective" modifer.