Concave polygon fill and the plot widget

From what I can tell by reading the docs/playing with the code currently only convex polygon filling is supported. Is supporting concave (or possibly even complex polygons) a future goal for egui or is that outside the scope of the project? I think something like [Lyon](https://github.com/nical/lyon) could be used for tessellation, and I saw the [Mesh](https://docs.rs/epaint/0.13.0/epaint/struct.Mesh.html) type so I can probably combine the two, but I didn't see support for the mesh type with the plot widget.

I really like the egui API in general and the recent improvements to the plot widget are very nice. I would like to be able to add concave polygons to plots. I am currently using wasm + html/js to create the [cavalier contours](https://github.com/jbuckmccready/cavalier_contours) web demo (web demo link [here](https://jbuckmccready.github.io/cavalier_contours_web_demo_page/)), but the egui API looks very nice and keeps everything in Rust.

I looked briefly at the code and I see the plot widget structs use the shape types/rendering under the hood. One option I see is just adding generic mesh object support to the plot widget (kind of a low level interface for the widget), another possibility is building out support for concave polygons in general, any thoughts/plans already around this?

Read the plot widget structs and the shape/rendering types mentioned in the issue, then compare the existing convex polygon path with epaint::Mesh and the proposed Lyon tessellation approach. Done means the project has resolved whether to support generic meshes or concave polygons and has a defined plot-widget API and rendering behavior for the chosen scope.

Written by the indexing model from the issue text.