Returns a canvas that draws on the page, after its existing content; dispose it to store what was drawn.
public PdfCanvas CreateCanvas()
ISO 32000-2 §7.8.2, §8.2 to §8.6; Table 31 (Contents, Resources). The new content goes into a new stream added to
the page's Contents (existing content wrapped in q and Q); resources go into the page's resource dictionary
(the inherited one when the page has none of its own, which other pages sharing it then also hold, unused; a new one when there
is none). See PdfCanvas. A mutation.