fdo#74424 Use Window::GetOutDev() to access ImplSelectClipRegion()

Part of the decoupling of Window from OutputDevice. We now get
the Window's OutputDevice instance and manipulate this. Do not rely
on the inherited function.

Conflicts:
	vcl/source/window/window.cxx

Change-Id: Idae6dcaa1c774a437887a7cfdbfc882aa4b35325
Reviewed-on: https://gerrit.libreoffice.org/7797
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>