[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

plugins/platforms/x11/windowed/x11windowed_backend.cpp

@@ -371,6 +371,7 @@ void X11WindowedBackend::updateSize(xcb_configure_notify_event_t *event)
     if (s != (*it).size) {
         (*it).size = s;
         int x = (*it).internalPosition.x() + s.width();
+        it++;
         for (; it != m_windows.end(); ++it) {
             (*it).internalPosition.setX(x);
             x += (*it).size.width();