Expose minimum and maximum zoom for clusters

[tmcw]

mapbox-gl-js version: latest

Steps to Trigger Behavior

1. Design a map that uses clusters
2. Intend to zoom to a level where a given cluster is expanded to show points
3. While we have a property for how many points are in a cluster, we don't have one for this.

Expected Behavior

Clusters contain a min_zoom property and max_zoom for the bounds at which they are clustered.

Actual Behavior

They don't

---

Implementation-wise, supercluster's getClusterProperties method should expose the value of the cluster's max zoom. Behind that, clusters have a zoom property but this is the last zoom at which they are processed: this will need to include their range as well.

[mourner]

Why do you need these properties? Something like "click to expand" that would zoom in by one level in most cases but sometimes more?

[tmcw]

Something like "click to expand" that would zoom in by one level in most cases but sometimes more?

Yep, that'd be one of the primary usecases - @arunasank can go into more detail about how this number would be useful for her project.

[arunasank]

Something like "click to expand" that would zoom in by one level in most cases but sometimes more?

@mourner Exactly. If we had a property to reveal the exact zoom at which a cluster splits, it'll be easy to just split on click sometimes.

It's useful when the points that make a cluster are very very close to each other sometimes, and spread apart at others.

I have some clusters on http://arunasank.github.io/labhopping. The cluster on the bottom left splits at zoom 6, but the one on the top right only splits at zoom level 19 or so.

[mourner]

@arunasank Sounds good! I'll look into it after vacation. Click to expand would be for the cluster max zoom, but what use case do you have for min zoom?

[arunasank]

@mourner \o/ The cluster max zoom would be super useful for our project, thanks! : )

I can't think of many use cases for the min zoom besides

• Click to zoom back out into cluster-view if you're in non-cluster-view.
• Instead of smooth zoom-in and zoom-out, zoom-in and zoom-out to have only two views - cluster or non-cluster by using your current cursor location to center the map.

[mourner]

Added in #6829