Fix a rendering glitch caused by the reflections in CoverSwitch

Seems like we need to use drawWindow instead of paintWindow or
KWin will keep some parts of the paint data.

BUG: 276810
FIXED-IN: 4.7.0

effects/coverswitch/coverswitch.cpp

@@ -752,7 +752,7 @@ void CoverSwitchEffect::paintWindowCover(EffectWindow* w, bool reflectedWindow,
             } else if (stop) {
                 data.opacity *= 1.0 - timeLine.currentValue();
             }
-            effects->paintWindow(w,
+            effects->drawWindow(w,
                                  PAINT_WINDOW_TRANSFORMED,
                                  infiniteRegion(), data);
             shader->setUniform("screenTransformation", origMatrix);