Support multiple PDF rendering backends in the GM

gm/tests/outputs/add-config-pdf/output-expected/json-summary.txt

 {
    "actual-results" : {
       "failed" : null,
       "failure-ignored" : null,
-      "no-comparison" : null,
+      "no-comparison" : {
+         "pdf/selftest1.png" : [ "bitmap-64bitMD5", 1149339852105949057 ]
+      },
       "succeeded" : {
          "565/selftest1.png" : [ "bitmap-64bitMD5", 12927999507540085554 ],
          "8888/selftest1.png" : [ "bitmap-64bitMD5", 1209453360120438698 ]
       }
    },
    "expected-results" : {
       "565/selftest1.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 12927999507540085554 ]
          ],
          "ignore-failure" : false
       },
       "8888/selftest1.png" : {
          "allowed-digests" : [
             [ "bitmap-64bitMD5", 1209453360120438698 ]
          ],
          "ignore-failure" : false
+      },
+      "pdf/selftest1.png" : {
+         "allowed-digests" : null,
+         "ignore-failure" : false
       }
    }
 }