[decorations] Don't recreate Renderer once the Compositor is destroyed

If we get a compositingToggled because the Compositor is going down we
don't need to recreate the Renderer as KWin as a whole is going down.

Thus we disconnect the compositingToggled connection when the Compositor
is about to be destroyed.

decorations/decoratedclient.cpp

@@ -75,7 +75,7 @@ DecoratedClientImpl::DecoratedClientImpl(AbstractClient *client, KDecoration2::D
             &Decoration::DecoratedClientImpl::signalShadeChange);
     connect(client, &AbstractClient::keepAboveChanged, decoratedClient, &KDecoration2::DecoratedClient::keepAboveChanged);
     connect(client, &AbstractClient::keepBelowChanged, decoratedClient, &KDecoration2::DecoratedClient::keepBelowChanged);
-    connect(Compositor::self(), &Compositor::compositingToggled, this,
+    m_compositorToggledConnection = connect(Compositor::self(), &Compositor::compositingToggled, this,
         [this, decoration]() {
             delete m_renderer;
             m_renderer = nullptr;
@@ -83,6 +83,12 @@ DecoratedClientImpl::DecoratedClientImpl(AbstractClient *client, KDecoration2::D
             decoration->update();
         }
     );
+    connect(Compositor::self(), &Compositor::aboutToDestroy, this,
+        [this] {
+            disconnect(m_compositorToggledConnection);
+            m_compositorToggledConnection = QMetaObject::Connection();
+        }
+    );
     connect(client, &AbstractClient::quickTileModeChanged, decoratedClient,
         [this, decoratedClient]() {
             emit decoratedClient->adjacentScreenEdgesChanged(adjacentScreenEdges());

decorations/decoratedclient.h

@@ -99,6 +99,7 @@ private:
     void createRenderer();
     AbstractClient *m_client;
     Renderer *m_renderer;
+    QMetaObject::Connection m_compositorToggledConnection;
 };
 
 }