Add an actual sphere.

As mentioned in js/shape.js:

 // Safari does not render lines with no size, have to render circle instead 

Some renderers don't like single-point paths with strokes on them. Most do seem to be fine with it, but I've run into a few crappy "SVG to PNG" converters online that won't render them, and Adobe Illustrator refuses to play nice with them, too.

The simplicity of spheres just being the most basic kind of single-point Shape is nice, but I don't think that's ruined by having a sphere just be a circle. Plus that'd open up an opportunity for having separate strokes and fills, and that seems nifty. So let's just render with an SVG circle rather than a path.