Don't add subpixel accumulations for squashed layers when invalidating them.

LayoutTests/compositing/squashing/invalidation-for-subpixel-offset-of-squashed-layer-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "shouldFlattenTransform": false,
           "children": [
             {
               "position": [8, 8],
               "bounds": [200, 200],
               "contentsOpaque": true,
               "drawsContent": true,
-              "backgroundColor": "#ADD8E6"
+              "backgroundColor": "#D3D3D3"
             },
             {
               "position": [50, 50],
-              "bounds": [50, 50],
+              "bounds": [251, 251],
               "drawsContent": true,
               "repaintRects": [
-                [0, 0, 50, 50],
-                [0, 0, 40, 40]
+                [50, 50, 200, 200]

[chrishtr, 2014/10/14 21:43:13]

Before this fix it was [51, 51, ...]

               ]
             }
           ]
         }
       ]
     }
   ]
 }