Add virtual Scene::scenePainter method
Summary:
So far EffectsHandlerImpl directly accessed SceneQPainter::painter
through a dynamic cast. If in future the QPainter based compositor should
be moved into a plugin we cannot access it through a dynamic cast.
To solve this problem the painter method is moved into Scene as
a virtual method returning a sane default value.
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D7214