[effects/translucency] Cancel existing animations before starting new

Summary:
It can happen that startAnimation is invoked multiple times for a
window. In case it was invoked a second time the previous animation was
not cancelled. This resulted in the set-animation to never end. When
closing a window, it would stay around as a translucent, non-interactive
window zombie.

This change ensures that existing animations get cancelled.

BUG: 342716
FIXED-IN: 5.8.3

Test Plan: Tested through autotest and manually.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3190

autotests/integration/effects/translucency_test.cpp

@@ -238,12 +238,9 @@ void TranslucencyTest::testDialogClose()
     QVERIFY(windowDeletedSpy.isValid());
     QVERIFY(windowClosedSpy.wait());
     if (windowDeletedSpy.isEmpty()) {
-        QEXPECT_FAIL("", "BUG 342716", Continue);
         QVERIFY(windowDeletedSpy.wait());
     }
-    QEXPECT_FAIL("", "BUG 342716", Continue);
     QCOMPARE(windowDeletedSpy.count(), 1);
-    QEXPECT_FAIL("", "BUG 342716", Continue);
     QTRY_VERIFY(!m_translucencyEffect->isActive());
     xcb_destroy_window(c.data(), w);
     c.reset();

effects/translucency/package/contents/code/main.js

@@ -78,6 +78,9 @@ var translucencyEffect = {
                         to: value / 100.0
                     }]
                 });
+                if (window.translucencyWindowTypeAnimation !== undefined) {
+                    cancel(window.translucencyWindowTypeAnimation);
+                }
                 window.translucencyWindowTypeAnimation = ids;
             }
         };