Define geometry construction api, e.g. circle, rectangle etc.

[prushforth]

This came up as a comment about MapML lacking a circle geometry, but given that a circle can be modeled by a Polygon, it occurs to me that rather than go down the path of adding geometry objects to MapML, we should define an API for "popular" geometries, allowing the creation of such MapML features programatically. The original question is here. This is a sub-issue of #106

[prushforth]

I believe this to be in the domain of what should be considered progressive enhancement, and it is well within the scope of using <svg> content in a map layer, so closing for now. Reopen as required.;