fix: supercluster max zoom with fractional zooming

[scarg0e]

Whether to re-cluster on zoom is determined by the fractional zoom value here:

js-markerclusterer/src/algorithms/supercluster.ts

Line 76 in c5cdf54

if (this.state.zoom <= this.maxZoom || state.zoom <= this.maxZoom) {

however the clusters are calculated using a rounded integer value here:

js-markerclusterer/src/algorithms/supercluster.ts

Line 92 in c5cdf54

.getClusters([-180, -90, 180, 90], Math.round(map.getZoom()))

This means if our new zoom value is fractionally above our maxZoom, it will be the final re-cluster on zooming in. However the clusters returned will not be de-clustered, as the rounded down value used for getClusters is not above our maxZoom, leaving our clusters not in the final de-clustered state even as we continue to zoom in.

Fixes #647 🦕

example of existing behaviour from issue 647:

maxZoom=14
zoom map to 14.2
getClusters called for zoom=14
zoom map to 16.2
getClusters not called
Result: will never decluster as getClusters is never called with a value above maxZoom

example of behavior with fix:

maxZoom=14
zoom map to 14.2
getClusters called for zoom=14
zoom map to 16.2
getClusters called for zoom=16
Result: does decluster as getClusters is called with a value above maxZoom

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