recursive tile loading for 404s doesn't work for dem tiles

@stepankuzmin is already working on this.

Since Nov 10, 2021.

• #11276 by @stepankuzmin — merged

If the terrain dem tileset is sparse (tiles don't go up to the full maxzoom in all areas), terrain tiles can fail to load. Terrain only loads after a bunch of panning or zooming. Discovered through https://github.com/mapbox/mapbox-gl-js/issues/11153

**mapbox-gl-js version**: main

**browser**: all

### Steps to Trigger Behavior

1. add a sparse dem tile source
2. load the map in an area where the tiles don't go up to the max zoom level with a zoom greater than the max zoom that returns tiles.

### Link to Demonstration

// internal

### Expected Behavior

Terrain loads.

### Actual Behavior

Terrain only loads after a bunch of panning.

---

If a tile 404s, we call `sourceCache.update(...)` to continue looking for a parent tile:
https://github.com/mapbox/mapbox-gl-js/blob/f919053d277cbb55ad1bafb76e7f1d7d5f94f486/src/source/source_cache.js#L242-L244

But we skip the update for terrain tiles (as an optimization, not intended for this situtation):
https://github.com/mapbox/mapbox-gl-js/blob/f919053d277cbb55ad1bafb76e7f1d7d5f94f486/src/source/source_cache.js#L472-L475

If we pass the right flag to `update(...)`, tiles will load successfully.

If we want to rely on recursive tile loading we need to review it carefully and add more tests. I don't think we've relied on this before, so I'm not confident there aren't other bugs.