QIconLoader: clear cache when the key becomes invalid

Change-Id: I6f2745715b902ccbc87d78b1c90f6883cfdd76ae
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit e2f0495e7efffefe927e43b26fa3a3a610aa003f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/image/qiconloader.cpp

@@ -135,8 +135,9 @@ void QIconLoader::invalidateKey()
 {
     // Invalidating the key here will result in QThemeIconEngine
     // recreating the actual engine the next time the icon is used.
-    // We don't need to clear the QIcon cache itself.
     m_themeKey++;
+    // Since the key has changed, we need to clear the cache as well.
+    QIconPrivate::clearIconCache();
 }
 
 QString QIconLoader::themeName() const