Add support for general polygons.

[tshead2]

Actually, more like polygons or squishy blobs that are the convex hull of a collection of points. This would facilitate analyzing clusters, both in graphs and scatterplots.

• [ ] Implement a polygon mark type. Abusing the plot mark type doesn't make sense because plots are open and polygons are closed. Also, having multiple series doesn't make sense for polygons.
• [ ] Should it be just straight lines, or splines, or ???
• [ ] Provide functionality to create a polygon mark using a collection of points and a boundary distance.
• [ ] It would be nice if the boundary distance could be constant in canvas space instead of domain space.