Fix an issue with small-radius clusters and uncap clusterMaxZoom

[mourner]

Closes #2898. Closes #6454. There were two issues here:

• Close points sometimes not being clustered on max zooms with small cluster radius — a tricky numerical bug fixed by Fix small-radius clusters not always forming on higher zooms supercluster#173
• clusterMaxZoom was always capped by source.maxzoom - 1, which is not always desirable. This PR changes behavior so that the option is respected without capping (although it's still maxzoom - 1 by default if not set). This means that you can set it in such a way that clusters occur at all zoom levels, including the max one, but this is no longer an issue because you can still recover original points if needed with source.getClusterLeaves, and we leave the decision whether to force the last zoom level to be unclustered to the user.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality (covered in Supercluster)
• document any changes to public APIs (the clusterMaxZoom behavior now matches the existing docs more closely)
• manually test the debug page
• tagged @zmiao to check whether the clusterMaxZoom change needs to be reflected in native
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fix a bug where close points where sometimes not clustered on higher zoom levels given a small clustering radius. Fix clusterMaxZoom so that it's not capped by source maxzoom.</changelog>

[asheemmamoowala]

This PR changes behavior so that the option is respected without capping (although it's still maxzoom - 1 by default if not set). This means that you can set it in such a way that clusters occur at all zoom levels, including the max one, but this is no longer an issue because you can still recover original points if needed with source.getClusterLeaves

What do you think about adding a query+render test to assert this new behavior?

[zmiao]

@mourner thanks for tagging me regarding this issue.
After checking on gl-native side, it turns out we are behaving similarly as how the fix is doing, i.e. taking the clusterMaxZoom with its original value if it is set, but with one difference from gl-js, that is if undefined, use default value uint8_t clusterMaxZoom = 17;, as the default max zoom is uint8_t maxzoom = 18; But I think we should update it to use option.maxzoom - 1 instead of 17.

Here is the source code link.