getCurrentViewPortBox should not use canvas height / width, breaks supercluster

[raysuelzer]

When changing the zoom level in the browser, the canvas height / width no longer accurately describes the accurate viewport box of what objects should be visible.

See: https://github.com/stepankuzmin/mapbox-gl-js/blob/master/src/ui/map.js#L1523

The result of this is that when markers or clusters are added to the maps, if the user changes the zoom level of the browser, then markers disappear from the map when they should still be visible.

For example, go to the demo here:
https://wykks.github.io/ngx-mapbox-gl/demo/ngx-marker-cluster
and set the zoom on the browser to 50%, you will see half the of the clusters completely disappear.

In order to get an accurate representation of the viewport box, the style property should be used to get the viewport box.

https://github.com/Wykks/ngx-mapbox-gl/blob/master/projects/ngx-mapbox-gl/src/lib/map/map.service.ts#L476-L477

[Wykks]

Indeed! I'll fix that asap :)