Properly clip the damage region and windows in SceneQPainter

Summary:
On the framebuffer backend the software rendered cursor was creating
rendering artifacts due to incorrect clipping. This change ensures that
in a single screen setup the clipping is performed correctly. For multi
screen a similar change might be required, but we currently don't have
any a way to test that as framebuffer only supports one screen and other
(visual) platforms don't support the software cursor.

CCBUG: 356328

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2024