Use scratch keys for stencil buffers.

expectations/gm/Test-Ubuntu12-ShuttleA-GTX660-x86-Debug/expected-results.json

 {
   "actual-results": {
     "failed": null, 
     "failure-ignored": null, 
     "no-comparison": null, 
     "succeeded": null
   }, 
   "expected-results": {
     "3x3bitmaprect_565.png": {
       "allowed-digests": [
@@ skipping 7041 matching lines @@
           "bitmap-64bitMD5", 
           17074122650246637487
         ]
       ], 
       "reviewed-by-human": true
     }, 
     "dashing2_nvprmsaa4.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
-          10566742216990551435
+          11009857797348161863
         ]
       ], 
       "reviewed-by-human": true
     }, 
     "dashing2_pdf-poppler.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           10758207538337005238
         ]
@@ skipping 14928 matching lines @@
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           11835374726311942978
         ]
       ], 
       "reviewed-by-human": true
     }
   }
 }