Revert "Reset buffer swap state on stop"

This reverts commit ba2c0324d2.

See: https://mail.kde.org/pipermail/kwin/2020-January/002999.html

composite.cpp

@@ -404,7 +404,7 @@ void Compositor::scheduleRepaint()
         //       in AnimationEffect::postPaintScreen. Why?
         //       Theory is that effects call addRepaintFull in there and then performCompositing
         //       is called again while still in the first paint. So queing it here makes sense!
-        compositeTimer.start(0, this);
+        QTimer::singleShot(0, this, [this]() { performCompositing(); });
     } else {
         setCompositeTimer();
     }
@@ -471,8 +471,6 @@ void Compositor::stop()
 
     delete m_scene;
     m_scene = nullptr;
-    m_bufferSwapPending = false;
-    m_composeAtSwapCompletion = false;
     compositeTimer.stop();
     repaints_region = QRegion();
 
@@ -741,8 +739,11 @@ void Compositor::performCompositing()
     // is called the next time. If there would be nothing pending, it will not restart the timer and
     // scheduleRepaint() would restart it again somewhen later, called from functions that
     // would again add something pending.
-    Q_ASSERT(!m_bufferSwapPending);
-    scheduleRepaint();
+    if (m_bufferSwapPending) {
+        m_composeAtSwapCompletion = true;
+    } else {
+        scheduleRepaint();
+    }
 }
 
 template <class T>