Returns a canvas that draws in the form, after its existing content; dispose it to store what was drawn.
public PdfCanvas CreateCanvas()
ISO 32000-2 §8.10.1, Table 93, and §8.2 to §8.6. The form's stream is replaced by its decoded content wrapped in q and
Q followed by what was drawn, unfiltered (a save compresses it); resources go into the form's Resources, created
when missing. Every use of the form shows the change. See PdfCanvas. A mutation.