Convert a rectangle r to SVG path commands. r is an object of the form { x: [number], y: [number], width: [number], height: [number], top-ry: [number], top-ry: [number], bottom-rx: [number], bottom-ry: [number] } where x, y, width, height are the usual rectangle attributes and [top-/bottom-]rx/ry allows for specifying radius of the rectangle for all its sides (as opposed to the built-in SVG rectangle that has only rx and ry attributes).