Compute correct position of drag button press on X11

Summary:
While dimensions of QScreen are in logical pixels, the origin is not.
As result, computed native coordinates are incorrect on high dpi.

Test Plan:
Drag a window by empty area on a multiple monitor setup (with High DPI),
neither cursor nor windows should "jump."

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20182