Fix merging annotations with compressed fallback layers

[erjel]

Description:

If an annotation is merged with a compressed fallback layer, the bounding box of the output_mag needs to account for compressed shard writes outside the original bounding box beforehand. If the bounding box is not adapted accordingly prior to concurrent calls of merge_chunk, the merge operation will corrupt the target dataset properties and result in a JSONDecodeError.

Todos:

Make sure to delete unnecessary points or to check all before merging:

• [x] Updated Changelog
• [x] Updated Documentation -> not applicable
• [x] Added / Updated Tests
• [x] Considered adding this to the Examples -> not applicable