Add virtual Scene::xrenderBufferPicture method

Summary:
So far EffectsHandlerImpl directly accessed SceneXrender::bufferPicture
through a dynamic cast. If in future the XRender based compositor should
be moved into a plugin we cannot access it through a dynamic cast.

To solve this problem the bufferPicture method is moved into Scene as
a virtual method returning a sane default value.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7207