Gets the colour key ranges: a minimum and maximum raw sample value per component; a sample whose every component is in its range is not painted. Empty when the image has none.
public ReadOnlySpan<double> ColorKey { get; }
ISO 32000-2 §8.9.6.4 (Mask as an array): compared with the raw samples, before DecodeMap; for an Indexed
image the values are indices. ColorKey(ReadOnlySpan<UInt16>, Int32, ReadOnlySpan<Double>, Span<Byte>) computes the coverage of a row.