x11: Properly detect compositing status

When finishing compositing, Workspace::compositing() will return true,
but it will be preferred if it returns false instead so kwin can properly
update x11 window visibility status, etc.

Instead of checking whether the compositor has a scene, check the status
of the compositor. It will not be "On" during teardown.

src/toplevel.cpp

@@ -285,10 +285,7 @@ void Toplevel::finishCompositing(ReleaseReason)
 
 bool Toplevel::compositing() const
 {
-    if (!Workspace::self()) {
-        return false;
-    }
-    return Workspace::self()->compositing();
+    return Compositor::compositing();
 }
 
 void Toplevel::addRepaint(const QRect &rect)

src/workspace.cpp

@@ -1891,7 +1891,7 @@ Toplevel *Workspace::findInternal(QWindow *w) const
 
 bool Workspace::compositing() const
 {
-    return m_compositor && m_compositor->scene();
+    return Compositor::compositing();
 }
 
 void Workspace::markXStackingOrderAsDirty()