[Effects/blend] Let fullscreen effects take priority

The Blendchanges effect works by showing a snapshot of the current
screen for a few ms. Any other active full screen effect will look
broken, so it should take priority.

This is pre-empting an issue with a new Plasma feature that syncs
colours to wallpapers. Once we animate activity switches / have per
desktop wallpapers it'll glitch out.

src/effects/blendchanges/blendchanges.cpp

@@ -41,6 +41,10 @@ void KWin::BlendChanges::start(int delay)
     if (!supported() || m_state != Off) {
         return;
     }
+    if (effects->hasActiveFullScreenEffect()) {
+        return;
+    }
+
     const EffectWindowList allWindows = effects->stackingOrder();
     for (auto window : allWindows) {
         redirect(window);