It actually changes it into a VirtualDevice and should just be a
refactoring. We get rid of the crude stuff in a follow up patch,

While at it unfriend PDFWriterImpl from OutputDevice.

Change-Id: Id43731ad076690292c30f9f3e05ff0dd58edc5e5
Reviewed-on: https://gerrit.libreoffice.org/62201
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>