rebaseline_server: use just skpdiff, not Python Image Library

gm/rebaseline_server/testdata/outputs/expected/compare_to_expectations_test.CompareToExpectationsTest.test_gm/gm.json

Index: gm/rebaseline_server/testdata/outputs/expected/compare_to_expectations_test.CompareToExpectationsTest.test_gm/gm.json
diff --git a/gm/rebaseline_server/testdata/outputs/expected/compare_to_expectations_test.CompareToExpectationsTest.test_gm/gm.json b/gm/rebaseline_server/testdata/outputs/expected/compare_to_expectations_test.CompareToExpectationsTest.test_gm/gm.json
index f2f40d507891aab6fecf67534d02f801f7d34ef8..1e3d947645ada779155253972a97d451eea9ba0a 100644
--- a/gm/rebaseline_server/testdata/outputs/expected/compare_to_expectations_test.CompareToExpectationsTest.test_gm/gm.json
+++ b/gm/rebaseline_server/testdata/outputs/expected/compare_to_expectations_test.CompareToExpectationsTest.test_gm/gm.json
@@ -110,7 +110,7 @@
     }
   }, 
   "header": {
-    "dataHash": "7849962375815855931", 
+    "dataHash": "-7804718549064096650", 
     "isEditable": false, 
     "isExported": true, 
     "schemaVersion": 3, 
@@ -128,7 +128,7 @@
         ], 
         "numDifferingPixels": 120000, 
         "percentDifferingPixels": 75.0, 
-        "perceptualDifference": 50.122499999999995
+        "perceptualDifference": 50.122499
       }, 
       "expectations": {
         "bugs": null, 
@@ -154,7 +154,7 @@
         ], 
         "numDifferingPixels": 765891, 
         "percentDifferingPixels": 97.38807678222656, 
-        "perceptualDifference": 25.25699999999999
+        "perceptualDifference": 25.256985
       }, 
       "expectations": {
         "bugs": [
@@ -182,7 +182,7 @@
         ], 
         "numDifferingPixels": 422432, 
         "percentDifferingPixels": 53.715006510416664, 
-        "perceptualDifference": 25.120500000000007
+        "perceptualDifference": 25.120543999999995
       }, 
       "expectations": {
         "bugs": [
@@ -202,16 +202,6 @@
       "isDifferent": true
     }, 
     {
-      "differenceData": {

[epoger, 2014/06/12 07:02:07]

I figured out that the two missing differenceData chunks in this unittest
correspond to image pairs with mismatched sizes.  skpdiff totally barfs (like,
totally) when you give it a pair of images with different sizes.

So we get different behavior for mismatched-size pairs now.  But I'm not sure
whether the old or new behavior is better anyway.

[djsollen, 2014/06/12 13:27:00]

We should update skpdiff to fail gracefully. File a bug if you want me to look
into it.

[epoger, 2014/06/12 14:02:34]

It's actually already pretty clear when skpdiff failed (not so clear WHY it
failed--image dimension mismatch, or what).  Yes, I think skpdiff could stand to
be improved a bit in that department but I think it's decent already.

The good news is that the end result is fine.  If you diff two images with
differing dimensions, you're gonna have a bad time.

-        "maxDiffPerChannel": [
-          222, 
-          223, 
-          222
-        ], 
-        "numDifferingPixels": 53150, 
-        "percentDifferingPixels": 12.035778985507246, 
-        "perceptualDifference": 100
-      }, 
       "expectations": {
         "bugs": [
           1578
@@ -230,16 +220,6 @@
       "isDifferent": true
     }, 
     {
-      "differenceData": {
-        "maxDiffPerChannel": [
-          221, 
-          221, 
-          221
-        ], 
-        "numDifferingPixels": 53773, 
-        "percentDifferingPixels": 12.17685688405797, 
-        "perceptualDifference": 100
-      }, 
       "expectations": {
         "bugs": [
           1578