Pass creation of Scene::EffectFrame to the Scene

Instead of having the EffectFrameImpl check the compositor type and do
the decision which Scene::EffectFrame to create, a pure virtual method
in Scene is called which returns the specific Scene::EffectFrame.

effects.cpp

@@ -31,7 +31,6 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include "cursor.h"
 #include "group.h"
 #include "scene_xrender.h"
-#include "scene_opengl.h"
 #include "unmanaged.h"
 #ifdef KWIN_BUILD_TABBOX
 #include "tabbox.h"
@@ -1891,16 +1890,7 @@ EffectFrameImpl::EffectFrameImpl(EffectFrameStyle style, bool staticSize, QPoint
     m_selection.setCacheAllRenderedFrames(true);
     m_selection.setEnabledBorders(Plasma::FrameSvg::AllBorders);
 
-    if (effects->isOpenGLCompositing()) {
-        m_sceneFrame = new SceneOpenGL::EffectFrame(this, static_cast<SceneOpenGL*>(Compositor::self()->scene()));
-    } else if (effects->compositingType() == XRenderCompositing) {
-#ifdef KWIN_HAVE_XRENDER_COMPOSITING
-        m_sceneFrame = new SceneXrender::EffectFrame(this);
-#endif
-    } else {
-        // that should not happen and will definitely crash!
-        m_sceneFrame = NULL;
-    }
+    m_sceneFrame = Compositor::self()->scene()->createEffectFrame(this);
 }
 
 EffectFrameImpl::~EffectFrameImpl()

scene.h

@@ -66,6 +66,12 @@ public:
 
     // a new window has been created
     virtual void windowAdded(Toplevel*) = 0;
+    /**
+     * @brief Creates the Scene backend of an EffectFrame.
+     *
+     * @param frame The EffectFrame this Scene::EffectFrame belongs to.
+     */
+    virtual Scene::EffectFrame *createEffectFrame(EffectFrameImpl *frame) = 0;
     /**
      * Method invoked when the screen geometry is changed.
      * Reimplementing classes should also invoke the parent method

scene_opengl.cpp

@@ -636,6 +636,11 @@ void SceneOpenGL::doneOpenGLContextCurrent()
     m_backend->doneCurrent();
 }
 
+Scene::EffectFrame *SceneOpenGL::createEffectFrame(EffectFrameImpl *frame)
+{
+    return new SceneOpenGL::EffectFrame(frame, this);
+}
+
 //****************************************
 // SceneOpenGL2
 //****************************************

scene_opengl.h

@@ -50,6 +50,7 @@ public:
     virtual qint64 paint(QRegion damage, ToplevelList windows);
     virtual void windowAdded(Toplevel*);
     virtual void windowDeleted(Deleted*);
+    virtual Scene::EffectFrame *createEffectFrame(EffectFrameImpl *frame);
     virtual void screenGeometryChanged(const QSize &size);
     virtual OverlayWindow *overlayWindow();
     virtual bool blocksForRetrace() const;

scene_xrender.cpp

@@ -288,6 +288,11 @@ void SceneXrender::windowAdded(Toplevel* c)
     windows[ c ]->updateShadow(c->shadow());
 }
 
+Scene::EffectFrame *SceneXrender::createEffectFrame(EffectFrameImpl *frame)
+{
+    return new SceneXrender::EffectFrame(frame);
+}
+
 //****************************************
 // SceneXrender::Window
 //****************************************

scene_xrender.h

@@ -45,6 +45,7 @@ public:
     virtual qint64 paint(QRegion damage, ToplevelList windows);
     virtual void windowAdded(Toplevel*);
     virtual void windowDeleted(Deleted*);
+    virtual Scene::EffectFrame *createEffectFrame(EffectFrameImpl *frame);
     virtual void screenGeometryChanged(const QSize &size);
     xcb_render_picture_t bufferPicture();
     virtual OverlayWindow *overlayWindow() {