Returns a canvas that draws the pattern's cell, after its existing content; dispose it to store what was drawn.
public PdfCanvas CreateCanvas()
ISO 32000-2 §8.7.3.1 to §8.7.3.3. The pattern'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 pattern's Resources, created when
missing. The cell of an uncoloured pattern (Uncolored) is a stencil: its canvas refuses colours,
pattern colours, shadings and the rendering intent with InvalidOperationException. This model is a snapshot: read
the pattern again (GetPattern(CosObject)) to see the new content. See PdfCanvas. A mutation.