Sets the soft mask that later painting is masked by (SMask through a graphics state parameter dictionary and gs),
or removes it (/SMask /None).
public void SetSoftMask(PdfSoftMask? mask)
The mask (CreateSoftMask(PdfSoftMaskType, PdfFormXObject, IReadOnlyList<Double>?, PdfFunction?), or one read from the document); null for none.
ISO 32000-2 §8.4.5, Table 57 (SMask, PDF 1.4), and §11.6.5.1. The mask's group is placed in the coordinate system of the
current transformation matrix at this call, concatenated with the group's own matrix: transform first, then set the mask. The
mask is part of the graphics state, so RestoreState() brings back the one in effect at SaveState(). Set
before DrawForm(PdfFormXObject) of a transparency group, it masks the group as a whole. Records PDF 1.4.