Gets the largest reduction, as a power of two, that GetSamplesAt(Int32) can deliver: 0 for an image decoded when the list was built, up to 3 (an eighth) or more for one whose codec decodes reduced resolutions.
public int MaxReduction { get; }
ISO 32000-2 §7.4.8 (DCT) and §7.4.9 (JPEG 2000 resolution levels): what the codec's header says it can do.