[platforms/x11] Properly reposition nested windows on resize events

Summary:
The position of the output being resized was adjusted instead of only
the other outputs.

Test Plan: Resized windows, verified xrandr

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2235