Originally I thought mpPDFWriter can be used to create PDF from
any OutputDevice, but it's actually just set for the internal
VirtualDevice of the PDF writer.

So this gets rid of all the special mpPDFWriter and GetPDFWriter()
handling and replaces it with checks for OUTDEV_PDF. But since
ImplPDFWriter used to be a OUTDEV_VIRDEV, this also introduces
OutputDevice::IsVirtual(), which now replaces most of the direct
OUTDEV_VIRDEV checks.

Change-Id: I11824143b6b8833ecc81119762448cbdf1145dbc
Reviewed-on: https://gerrit.libreoffice.org/62257
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>