[effects/diminactive] Delete active transitions when window is deleted

Summary:
The Dim Inactive effect expects that the windowClosed signal always
proceeds the windowDeleted signal. But in some cases that's not the case.

If a window gets destroyed before becoming ready for painting, only
the windowDeleted signal will be emitted. In addition to that, KWin will
activate that window, which means we'll probably start a transition for
it.

Because this effect cannot terminate active transitions for such
windows, KWin can crash in postPaintScreen.

This change addresses the crash in postPaintScreen by adding extra clean
up stuff in the windowDeleted slot to make sure that there are no
transitions for deleted windows.

The proper fix would be to not emit windowActivated signal for windows
that are not ready for painting.

BUG: 399612
FIXED-IN: 5.14.1

Test Plan:
Ran
```
x <- seq(5, 15, length=1000)
y <- dnorm(x, mean=10, sd=3)
plot(x, y, type="l", lwd=1)
```
in RKWard multiple times.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16130

effects/diminactive/diminactive.cpp

@@ -372,6 +372,12 @@ void DimInactiveEffect::windowClosed(EffectWindow *w)
 void DimInactiveEffect::windowDeleted(EffectWindow *w)
 {
     m_forceDim.remove(w);
+
+    // FIXME: Sometimes we can miss the window close signal because KWin
+    // can activate a window that is not ready for painting and the window
+    // gets destroyed immediately. So, we have to remove active transitions
+    // for that window here, otherwise we'll crash in postPaintScreen.
+    m_transitions.remove(w);
 }
 
 void DimInactiveEffect::activeFullScreenEffectChanged()