fdo#74424 - added TODO

The function OutputDevice::ImplInitClipRegion() should be moved into
Window, VirtualDevice and Printer. Added a TODO note.

Change-Id: Ia9c7a5299e8762ad96788232466b0772c0a2e6bb
Reviewed-on: https://gerrit.libreoffice.org/7796
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>