`points` needs to be called for polythings

[sjl]

When I try to run (draw scene (:polygon :points (list (list 0 0) (list 0 10) (list 10 0))) the resulting SVG is mangled because that list of points gets rendered as a Lisp list:

  <polygon points="((0 0) (0 10) (10 0))" fill="black"/>

I see there's a points function in the XML formatter, and if I call that on my list it works, but it's not external in the package. I'm not sure the best way to go about detecting when that needs to be called, otherwise I'd send a PR myself.