Ability to create filled polygon

Hi! I'm interested in creating a filled in polygon from a series of lines. I noticed rgx has the ability to create lines, so I could create the outline of a polygon from a series of lines, but not sure about filling it in. Is this currently possible? If it's possible, I'm willing to add an example to the `examples/` folder to demonstrate it!

Start by reviewing rgx's existing line-drawing API and the contents of the examples/ folder to determine whether filled polygons are already supported. The issue does not name an implementation entry point, test, or target API; done would require a decided approach for filled polygons and an example demonstrating the resulting behavior.

Written by the indexing model from the issue text.