As GetSamplesAt(Int32), stopping a decode in progress when cancellationToken is cancelled.
public DisplayImageSamples? GetSamplesAt(int reduction, CancellationToken cancellationToken)
The reduction asked for: 0 for the image's own resolution, 1 for half, 2 for a quarter, 3 for an eighth.
Given to the codec; a render's token, so that a cancelled tile does not wait for a long decode.
ISO 32000-2 §8.9.3 and §8.9.4; viewer capability 2 (cancellation).