Make Slideback work on OpenGL without PaintClipper

effects/slideback/slideback.cpp

@@ -204,6 +204,9 @@ void SlideBackEffect::paintWindow(EffectWindow *w, int mask, QRegion region, Win
     if (motionManager.isManaging(w)) {
         motionManager.apply(w, data);
     }
+    foreach (const QRegion &r, clippedRegions) {
+        region = region.intersected(r);
+    }
     effects->paintWindow(w, mask, region, data);
     for (int i = clippedRegions.count() - 1; i > -1; --i)
         PaintClipper::pop(clippedRegions.at(i));