Gets the colour the samples were pre-blended with (the soft mask's Matte), one value per component of the image's colour
space (of its base for Indexed); null when there is none or it cannot apply.
public IReadOnlyList<double>? Matte { get; }
ISO 32000-2 §11.6.5.2, Table 144 (PDF 1.4): undo it with Unpremultiply(Span<Single>, Int32, ReadOnlySpan<Single>, ReadOnlySpan<Single>, ReadOnlySpan<ComponentRange>). A Matte of the wrong length, or on a soft mask whose size differs from the image's (the samples would not line up), is ignored with a diagnostic.