Gets the colour the samples were pre-blended with, one value per component of ColorSpace (of its base for an Indexed space), to be removed with the SoftMask's opacity before conversion; empty when there is none.
public ReadOnlySpan<float> Matte { get; }
ISO 32000-2 §11.6.5.2, Table 144 (Matte): c = m + (c′ − m) / α (see Unpremultiply(Span<Single>, Int32, ReadOnlySpan<Single>, ReadOnlySpan<Single>, ReadOnlySpan<ComponentRange>)); the soft
mask then has the image's size. JPEG 2000 opacity premultiplied into the colour (SMaskInData 2, §8.9.5.1 Table 87) is
a matte of zeros.