Include layer postPaint in render loop timings

We need render timings in the journal to be as close as possible to the
entirity of the composite function up until the backend presenting.

src/composite.cpp

@@ -689,9 +689,9 @@ void Compositor::composite(RenderLoop *renderLoop)
             primaryLayer->endFrame(bufferDamage, surfaceDamage);
         }
     }
-    renderLoop->endFrame();
 
     postPaintPass(superLayer);
+    renderLoop->endFrame();
 
     m_backend->present(output);