Schlib: use polylines instead of rectangles
The rectangle graphics primitive is a legacy item. Newly saved symbols encode them as polylines. Add code to convert legacy rectangles into polylines. Add functions to check if a polyline is a rectangle. Add function to find the center rectangle of a symbol. Update rules S3.1 and S3.3 with the above functions.
Related: #334 (closed)
Edited by cpresser