Support in `read` for reading pixels outside of image bounds

We currently have a `boundless=True/False` param on `fetch_tile`, but this doesn't allow for fetching tiles that don't even exist in the source COG.

It could make downstream usage (for example in rio-tiler cc @vincentsarago ) if we allowed `read` outside of the pixel bounds of the image.

Start at the read entry point and compare its pixel-bound handling with fetch_tile's existing boundless parameter. Trace how requests for tiles outside the source COG are handled, then verify that read can return pixels beyond the image bounds, including regions with no source tile.

Written by the indexing model from the issue text.