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
2d01ba6450