[scenes/qpainter] Draw decoration shadows

Summary:
QPainter doesn't render decoration shadows. It renders only
shadows provided through ShadowInterface.

With this change, painting of shadows is done in similar way OpenGL backend is
currently doing.

Before

{F5734867, layout=center, size=full}

After

{F5734870, layout=center, size=full}

Depends on D10811 (dummy decoration with shadows in autotests)

Test Plan:
* start kwin with QPainter backend enabled:

```
KWIN_COMPOSE=Q kwin_wayland --xwayland --windowed
```

* open konsole and kate:

```
DISPLAY=:1 konsole
DISPLAY=:1 kate
```

Reviewers: #kwin, graesslin, davidedmundson

Reviewed By: davidedmundson

Subscribers: abetts, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D10943