Properly render XWayland windows in SceneQPainter

Summary:
The buffer for a XWayland window is larger than the actual size. Thus
we need to use the clientSize as reference, not the buffer size.

BUG: 382748

Test Plan: Test passes

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6952