fix jwilkes/purr-data#44

make [draw] arguments more closely follow the svg spec
* [draw circle 20 1 2]: radius=20, xcenter=1, ycenter=2
* [draw rect 5 10 20 40]: width=5, height=10, xoffset=20, yoffset=40
* [draw ellipse 5 10 20 40]: xradius=5, yradius=10, xcenter=20, ycenter=40