wrong tiles at low zooms with terrain enabled

Nobody has claimed this yet.

An unpitched map with terrain enabled shows a tiles from a mix of zoom levels. It should show tiles from the same zoom level (the way it looks without terrain). The visual effect is more severe when using tilesets with sharper cutoffs (data exists in z2, but not z1).

I think `transform.coveringTiles(...)` is not returning the right tiles and I think this may be related to the [distance calculation it uses](https://github.com/mapbox/mapbox-gl-js/blob/0eecef1d3fd42e9ebc41546e6ff1b292e9213491/src/geo/transform.js#L757). I think it uses a 3D distance from the camera point to the tile when maybe it should be using just the z distance from the camera to the tile.

**mapbox-gl-js version**: main (0eecef1)

**browser**: all

### Steps to Trigger Behavior

1. `yarn run start-debug`
2. open `http://localhost:9966/debug/terrain-debug.html#2.06/54.49/71.37` with a large viewport (innerWidth: 1701, innerHeight: 1361)
3. set `map.showTileBoundaries = true` in the console

### Expected Behavior

Viewport covered by z2 tiles:


### Actual Behavior

Viewport covered by a mix of z1 and z2 tiles:


Run `yarn run start-debug`, open the terrain debug URL from the report with a large viewport, and enable `map.showTileBoundaries`. Start in `src/geo/transform.js` at `transform.coveringTiles(...)` and its linked distance calculation; done means the viewport uses tiles from one zoom level, matching the no-terrain behavior.

Written by the indexing model from the issue text.