fdo#74424 Use Window::GetOutDev() to access ImplGetDPI(X|Y)()

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:
	include/vcl/window.hxx

Change-Id: I93c83578eb92cdf10c7009f98e91ab177879c141
Reviewed-on: https://gerrit.libreoffice.org/7788
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>