Gets which masking applies to the image.
public PdfImageMaskKind MaskKind { get; }
ISO 32000-2 §8.9.6, §11.6.4.3, Table 87: an image mask is a stencil; otherwise SMaskInData (non-zero, JPEG 2000 only)
wins over SMask, which wins over Mask. A Mask stream that is not an image mask, a non-stream SMask
and a colour key array of the wrong length are ignored with a diagnostic, as other readers do. Masks have no masks of their own.