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.
This should probably include F_REGULAR too - as you can't set the points of a regular graphic as they are defined by the other properties.