Add a number of tests for layer squashing

LayoutTests/compositing/squashing/squash-onto-transform-backing-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
       (children 1
         (GraphicsLayer
-          (position 8.00 8.00)
-          (bounds 784.00 90.00)
-          (isolate 1)
-          (contentsOpaque 1)
           (children 2
             (GraphicsLayer
-              (bounds 784.00 90.00)
+              (position 100.00 100.00)
+              (bounds 100.00 100.00)
               (contentsOpaque 1)
-              (drawsContent 1)
-              (backfaceVisibility hidden)
-              (backgroundColor #008000)
+              (transform [0.71 0.71 0.00 0.00] [-0.71 0.71 0.00 0.00] [0.00 0.00 1.00 0.00] [0.00 0.00 0.00 1.00])
             )
             (GraphicsLayer
-              (bounds 160.00 90.00)
-              (blendMode multiply)
+              (position 20.00 20.00)
+              (bounds 260.00 260.00)
+              (drawsContent 1)
             )
           )
         )
       )
     )
   )
 )