Gives the page a page group, writing << /Type /Group /S /Transparency >> as its Group, and returns
the group's attributes to set; returns the existing attributes when the page already has one.
public PdfTransparencyGroup CreateTransparencyGroup()
ISO 32000-2 §7.7.3.3, Table 31, §11.4.7 and §11.6.6, Table 145 (PDF 1.4). The page group is effectively isolated whatever IsIsolated says; its ColorSpace is honoured, and is the space the page's transparent content blends in. Records PDF 1.4. A mutation.