fdo#74702 GetPaintArea protected function introduced

GetPaintArea has been introduced for Window - we intersect with the
available paintable boundary to get the actual paintable area needed.
For OutputDevice, GetPaintArea just returns the same area.

Change-Id: If38b09eb54d516d189cedf8bba15b184e6b6f7d5
Reviewed-on: https://gerrit.libreoffice.org/8727
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>