Use scratch keys for stencil buffers.

expectations/gm/Test-Ubuntu12-ShuttleA-GTX660-x86_64-Debug/expected-results.json

 {
   "actual-results": {
     "failed": null, 
     "failure-ignored": null, 
     "no-comparison": null, 
     "succeeded": null
   }, 
   "expected-results": {
     "3x3bitmaprect_565.png": {
       "allowed-digests": [
@@ skipping 7113 matching lines @@
       ], 
       "bugs": [
         2325
       ], 
       "reviewed-by-human": false
     }, 
     "dashing2_nvprmsaa4.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
-          10566742216990551435
+          11009857797348161863
         ]
       ], 
-      "bugs": [
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-      ], 
-      "reviewed-by-human": false
+      "reviewed-by-human": true
     }, 
     "dashing2_pdf-poppler.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           4486503885534337283
         ]
       ], 
       "bugs": [
         1578
@@ skipping 15007 matching lines @@
       "allowed-digests": [
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           "bitmap-64bitMD5", 
           11835374726311942978
         ]
       ], 
       "reviewed-by-human": true
     }
   }
 }