Support LOD transition periods for offline tile selection

When #543 is merged, we will support a transition period where tiles from the original and target LOD are both kept alive. A parameter representing the fade percentage is swept throughout the period so that clients know how faded to render each tile. To achieve this evolving fade percentage, a new `deltaTime` parameter was added to `Tileset::updateView` to decide how much to change the fade percentage based on how much time had elapsed since the last frame.

However, the way `updateViewOffline` works is it repeatedly calls `updateView` until there are no tiles that are still loading. This seems awkward with the new `deltaTime` argument, since an individual call to `updateView` doesn't have a sensible delta time that could be associated with it. We should find a way to support lod transition periods for offline tile selection as well (e.g., a client might use it to smooth transitions in a high-res offline render). This might require one of the following approaches:
- Decouple the transition code from the traversal itself so that the traversal can run back-to-back several times and the transition can be applied afterwards. This might require managing further state information to track the result of not just a single traversal, but a sequence of traversals.
- Or we can reimplement `updateViewOffline` to work with a single BVH traversal, by parameterizing the traversal to "demand" the ideal resolution tiles by blocking and waiting, instead of kicking and "settling" for lower-res tiles.