[kcmkwin/kwindecoration] Properly render shadows with big tiles

Summary:
Decoration KCM module doesn't render properly shadows with big tiles.
This change tries to address that problem by clipping overlaps, similar
to the OpenGL backend.

Before

{F5734862, layout=center, size=full}

After

{F5734863, layout=center, size=full}

Test Plan:
* apply a given patch https://raw.githubusercontent.com/zzag/repo/sources/arch/breeze/refine-decoration-shadows.patch to breeze
* go to System Settings/Application Style/Window Decorations

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: abetts, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D10942