Return distance to feature from queryRenderedFeatures

Nobody has claimed this yet.

## Motivation

I'm mixing Mapbox GL fill-extrusion layers with some custom layers that use Three.js (see [this StackOverflow question/answer][1]). When a user clicks on the map, I can use Mapbox's `queryRenderedFeatures` to figure out which fill-extrusion features were under the cursor. And I can use a Three.js raycaster (see the Stack Overflow question) to figure out which Three.js features were under the cursor.

I want to sort these features by depth, so that I can determine which is on top, whether it's a fill-extrusion layer or a Three.js layer. The Three.js raycaster returns a distance which I can use to sort the meshes. And `queryRenderedFeatures` sorts the 3D features it returns by distance. But it doesn't return that distance. So it's not clear to me how to merge the two lists of features.

I believe Mapbox GL sorts 3D features by distance from the camera here: https://github.com/mapbox/mapbox-gl-js/blob/cc9e21926e8a498315c1a1bf75bf74702447b7f2/src/style/style.js#L1052-L1054

So my strawman proposal would be to expose `intersectionZ` (or some physically meaningful transformation of it) in the result of `queryRenderedFeatures`.

## Design Alternatives

A method to measure the distance from the camera to a feature would also solve this problem. Or if there's already a way to do that, I'd love to hear about it.

## Design

### Mock-Up

### Concepts

### Implementation

[1]: https://stackoverflow.com/questions/59163141/raycast-in-three-js-with-only-a-projection-matrix

Start with queryRenderedFeatures and the linked sorting code in src/style/style.js, then compare Mapbox GL's feature ordering with the distance returned by the Three.js raycaster. Done means the API exposes a physically meaningful distance, or an equivalent camera-distance method, so the two feature lists can be merged by depth.

Written by the indexing model from the issue text.