Use scratch keys for stencil buffers.

expectations/gm/Test-Win8-ShuttleA-GTX660-x86_64-Release/expected-results.json

 {
   "expected-results": {
     "3x3bitmaprect_565.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           12831071530564277917
         ]
       ], 
       "bugs": [
@@ skipping 7098 matching lines @@
       ], 
       "bugs": [
         1935
       ], 
       "reviewed-by-human": false
     }, 
     "dashing2_nvprmsaa4.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
-          10566742216990551435
+          11009857797348161863
         ]
       ], 
-      "reviewed-by-human": false
+      "reviewed-by-human": true
     }, 
     "dashing2_pdf-poppler.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           4486503885534337283
         ]
       ], 
       "bugs": [
         1935
@@ skipping 4771 matching lines @@
           "bitmap-64bitMD5", 
           9899853331722013852
         ]
       ], 
       "reviewed-by-human": false
     }, 
     "glyph_pos_n_b_msaa16.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
-          17326560431224736462
+          12821620961617303185
         ]
       ], 
-      "reviewed-by-human": false
+      "reviewed-by-human": true
     }, 
     "glyph_pos_n_b_nvprmsaa4.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           5595499012934038484
         ]
       ], 
       "reviewed-by-human": true
     }, 
@@ skipping 9631 matching lines @@
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           11835374726311942978
         ]
       ], 
       "reviewed-by-human": true
     }
   }
 }