Query tilesets by bounding volume at a particular LOD

`Tileset::updateView` effectively allows you to query for a set of tiles that meets a particular screen-space error from the viewpoint of a camera with a particular perspective frustum. After #1125, this will be extended to multiple independent views. And after #1135, it will be extended to orthographic views.

It would be useful to further extend this idea to allow querying of a set of tiles within a particular bounding box or sphere at a particular level-of-detail.

More generally, we should make it easy for users to implement these kinds of queries external to the `Tileset`, while still leveraging the progressive loading that `Tileset` does for views and height queries.

CC @Lee-Bull

Start by reading Tileset::updateView and the changes described in #1125 and #1135 to understand how view and orthographic queries work. Then examine how Tileset progressively loads data for views and height queries. Done means users can query tiles within a bounding box or sphere at a specified level of detail, while external queries can reuse that progressive loading.

Written by the indexing model from the issue text.