effects/blendchanges: ignore fullscreen windows

Fullscreen windows are most likely games or videos, where the blend effect is an
annoyance that freezes the screen for a moment when a accent color change is triggered
in the background.

src/effects/blendchanges/blendchanges.cpp

@@ -46,7 +46,9 @@ void KWin::BlendChanges::start(int delay)
 
     const EffectWindowList allWindows = effects->stackingOrder();
     for (auto window : allWindows) {
-        redirect(window);
+        if (!window->isFullScreen()) {
+            redirect(window);
+        }
     }
 
     QTimer::singleShot(delay, this, [this, animationDuration]() {