Support native MapCircle items in Mapbox GL plugin

Qt QML MapCircle radius property is measured in meters, while Mapbox GL's equivalent circle layer radius property is measured in pixels.

An open issue in Mapbox GL native discusses this parity issue, not only with Qt but also other vendors.

Currently we can workaround it by either rendering the circle as a polygon, or update each and every circle radius upon every zoom change, applying a conversion between projected meters and viewport pixels.