improve screen filtering for global menu applet

Summary:
--when the user has enabled PLASMA_USE_QT_SCALING
under X11 environment the screen and window geometries
can not be trusted for comparison before window
geometries have been adjusted properly

BUG: 404500
FIXED-IN: 5.15.2

Test Plan: check that global menu still works properly

Reviewers: #plasma, davidedmundson, broulik

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19126