Makes the form a transparency group XObject, writing << /Type /Group /S /Transparency >> as its
Group, and returns the group's attributes to set; returns the existing attributes when the form already is one.
public PdfTransparencyGroup CreateTransparencyGroup()
ISO 32000-2 §8.10.3, Table 94, and §11.6.6 (PDF 1.4). When the canvas paints the form (DrawForm(PdfFormXObject)), its
content composites as one group, with the alpha, blend mode and soft mask in effect at Do applied to the group as a whole;
inside it they start at 1, Normal and none. A group of another subtype is replaced. Records PDF 1.4. A mutation.