Issue repaints for RenderLayers that get new composited backings after

LayoutTests/compositing/squashing/squash-above-fixed-3-expected.txt

 This scenario verifies that the green "container" element and lime "innerScrolling" element scroll properly even though there is a blue fixed-position element layered in between them.
 
 The catch is that the squashing requirements should be computed in correct paint order, so that the green container does not accidentally position itself with respect to the wrong layer and not scroll.
 
 CASE 1, original layer tree:
 (GraphicsLayer
   (bounds 785.00 4100.00)
   (children 1
     (GraphicsLayer
       (bounds 785.00 4100.00)
@@ skipping 91 matching lines @@
           (contentsOpaque 1)
           (drawsContent 1)
           (backgroundColor #808080)
         )
         (GraphicsLayer
           (position 100.00 100.00)
           (bounds 200.00 4000.00)
           (drawsContent 1)
           (backgroundColor #008000)
           (repaint rects
+            (rect 100.00 0.00 100.00 100.00)
           )
           (children 1
             (GraphicsLayer
               (position 0.00 160.00)
               (bounds 200.00 100.00)
               (contentsOpaque 1)
               (drawsContent 1)
               (backgroundColor #0000FF)
             )
           )
         )
       )
     )
   )
 )