Set canvas size on addPage and don't destroy PDF surface on width/height change

[kayahr]

Changes explained:

• Moved cairo_pdf_surface_set_size from addPage method to setSize method of PDF backend. addPage now calls this setSize method so the canvas size is also changed.
• Changing width/height on canvas now calls the same setSize method so in PDF backend this also affects the PDF surface size.
• Changing width/height on canvas no longer destroyes the PDF surface, it only changes the page size and resets the context.

Fixes #2538

• Have you updated CHANGELOG.md?