Cluster of Polygon feature

Nobody has claimed this yet.

## Motivation

## Design

In my idea maybe we could represent a polygon with a point (center). And we could set this point in a property.

### Mock-Up

Something like this:
'properties': {
'anchor_polygon': [0, 0],
...
}

And it will use the same logic as what we have now, but instead of displaying/clustering point it would be polygons.

I don't know if my idea is too simple and if I'm missing stuff.

Thanks

The issue names no files, tests, or entry points. Start by reviewing the existing point-clustering behavior and how polygon features are represented, then define the required anchor and rendering behavior; done should include an agreed design and working polygon clustering with tests.

Written by the indexing model from the issue text.