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