Seperate initialization of OutputDevice instance

I have split OutputDevice::ImplInitGraphics() from
OutputDevice::ImplGetGraphics(). In future, we need to further
seperate out this initialization function into it's own seperate
classes. Note that ImplGetGraphics still initializes if mpGraphics
is not set.

Change-Id: I17d4778f735aa9d03bf9b37079a9d695bf95866b
Reviewed-on: https://gerrit.libreoffice.org/7777
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>