discard cache when composite changes

would make too many deletions?
another way could be including the compositing state in the cache key
but would be  a bit complex...

svn path=/trunk/KDE/kdelibs/; revision=883978

theme.cpp

@@ -163,6 +163,7 @@ void ThemePrivate::compositingChanged()
 
     if (compositingActive != nowCompositingActive) {
         compositingActive = nowCompositingActive;
+        pixmapCache->discard();
         emit q->themeChanged();
     }
 #endif