Allow 2D XY slices to progressively refine

## Use Case

- Load lower-resolution slice initially, then load in higher-resolution slice and swap.

## Solution

- How do we orchestrate the sequence of requests?
- How is this integrated with prefetching? (prefetch at single fixed resolution)
- Add an option to stagger a sequence of requests to the pre-fetching API (RequestQueue).
- Can be used for pre-fetching future frames without slowdowns when large amounts of data arrive at once, and can be interrupted by higher priority requests.