gm: report drawing mode discrepancies (e.g. pipe vs tiled) explicitly

gm/tests/outputs/pipe-playback-failure/output-expected/json-summary.txt

Index: gm/tests/outputs/pipe-playback-failure/output-expected/json-summary.txt
===================================================================
--- gm/tests/outputs/pipe-playback-failure/output-expected/json-summary.txt	(revision 0)
+++ gm/tests/outputs/pipe-playback-failure/output-expected/json-summary.txt	(revision 0)
@@ -0,0 +1,33 @@
+{
+   "actual-results" : {
+      "failed" : {
+         "comparison/selftest1-pipe" : {
+            "checksum" : 4259036727585789440
+         }
+      },
+      "failure-ignored" : null,
+      "no-comparison" : null,
+      "succeeded" : {
+         "565/selftest1" : {
+            "checksum" : 9512553915271796906
+         },
+         "8888/selftest1" : {
+            "checksum" : 14022967492765711532
+         }
+      }
+   },
+   "expected-results" : {
+      "565/selftest1" : {
+         "checksums" : [ 9512553915271796906 ],
+         "ignore-failure" : false
+      },
+      "8888/selftest1" : {
+         "checksums" : [ 14022967492765711532 ],
+         "ignore-failure" : false
+      },
+      "comparison/selftest1-pipe" : {
+         "checksums" : [ 14022967492765711532 ],
+         "ignore-failure" : false
+      }
+   }
+}