It's cleaner to avoid member functions and constructors when we inherit
from a C struct and we pass it to pdfium's C API.

Change-Id: Iac9ab99b3d1b66241bb49d3a7707e242078ab5df
Reviewed-on: https://gerrit.libreoffice.org/70472
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>