Consider refactor so that polygons have ownership over vertices

[adamconkey]

I originally designed things so that polygons did not own vertices. That was intentional so that multiple polygons could be created from the same set of vertices, and you'd have a common structure, VertexMap from which to retrieve them by name which would enable efficient retrieval. I realize now that may have been a mistake, it makes things more clunky when you want to load things from file, and creates more data structures than are probably necessary.

I'd like to at least try a version where ownership of vertices is on the polygons. If it feels more ergonomic in the code then I'll bring it to main and hope that implementing something else down the road doesn't necessitate something like my current setup 😅