Fix misplaced client windows

Summary:
This regression was introduced by me, sorry. The client window may be at
location other than (0, 0), in which case we have to move it to (0, 0)
to ensure that the client contents is not covered by window frame.

BUG: 417584
FIXED-IN: 5.18.1

Reviewers: KWin, broulik

Reviewed By: broulik

Subscribers: broulik, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D27456