[effects/blur] Lookup window once when disconnecting from blurChanged

Summary:
When disconnecting from `blurChanged` signal, `(w, connection)` pair is
being looked up three times. We can do better by using `QMap` in a more
STL-like way.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13118

effects/blur/blur.cpp

@@ -303,10 +303,12 @@ void BlurEffect::slotWindowAdded(EffectWindow *w)
 
 void BlurEffect::slotWindowDeleted(EffectWindow *w)
 {
-    if (windowBlurChangedConnections.contains(w)) {
-        disconnect(windowBlurChangedConnections[w]);
-        windowBlurChangedConnections.remove(w);
+    auto it = windowBlurChangedConnections.find(w);
+    if (it == windowBlurChangedConnections.end()) {
+        return;
     }
+    disconnect(*it);
+    windowBlurChangedConnections.erase(it);
 }
 
 void BlurEffect::slotPropertyNotify(EffectWindow *w, long atom)