Support edge insets in QGeoMap

QGeoMap should provide means of specifying that the map center / uncovered visible map rectangle differs from the whole visible map. In Mapbox GL Native, we apply edge insets (margins) on every side of the map view for that.

So far, Mapbox GL and Qt have been able to sync in terms of rendering because both projects use the same WebMercator projection logic. However, as Qt takes care of rendering e.g. MapQuickItem objects, and because Mapbox GL takes margins in consideration, the map objects rendered by Qt end up displaced. This displacement can be resolved if QGeoMap knows what these edge inset values are.