Shape coordinates
Each shape has a distinct `coords` grammar. A rectangle is `left,top,right,bottom`; a circle is `center-x,center-y,radius`; a polygon is a sequence of `x,y` vertex pairs. The origin is the image's top-left corner, x moves right, and y moves down. Polygon vertices should follow the boundary in order. The last point connects back to the first, so repeating the opening pair is unnecessary.