Ignore failures in gradtext_pdf-mac GM tests

expectations/gm/Test-Mac10.6-MacMini4.1-GeForce320M-x86_64-Release/expected-results.json

 {
   "actual-results": {
     "failed": null, 
     "failure-ignored": null, 
     "no-comparison": null, 
     "succeeded": null
   }, 
   "expected-results": {
     "3x3bitmaprect_565.png": {
       "allowed-digests": [
@@ skipping 7616 matching lines @@
       "ignore-failure": false
     }, 
     "gradtext_pdf-mac.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           2307761468478746873
         ]
       ], 
       "bugs": [
-        1578
+        476
       ], 
+      "ignore-failure": true, 
       "reviewed-by-human": false
     }, 
     "hairlines_565.png": {
       "allowed-digests": [
         [
           "bitmap-64bitMD5", 
           3746520695784445157
         ]
       ]
     }, 
@@ skipping 4666 matching lines @@
           10424380103329133154
         ]
       ], 
       "bugs": [
         1578
       ], 
       "reviewed-by-human": false
     }
   }
 }