skdiff: return nonzero exit value if there are any mismatches

This will allow us to replace "gm -r" in our buildbots with "skdiff".  This will speed things up, and work around http://code.google.com/p/skia/issues/detail?id=473 ('PDF gradtext gm image results are nondeterministic')
Review URL: https://codereview.appspot.com/6242071

git-svn-id: http://skia.googlecode.com/svn/trunk@4097 2bbb7eff-a529-9590-31e7-b0007b416f81

tools/skdiff_main.cpp

@@ -27,6 +27,8 @@
  * Creates an index.html in the current third directory to compare each
  * pair that does not match exactly.
  * Does *not* recursively descend directories.
+ *
+ * Returns zero exit code if all images match across baseDir and comparisonDir.
  */
 
 #if SK_BUILD_FOR_WIN32
@@ -1170,4 +1172,6 @@ int main (int argc, char ** argv) {
     }
     matchSubstrings.deleteAll();
     nomatchSubstrings.deleteAll();
+
+    return summary.fNumMismatches;
 }

tools/tests/skdiff/test1/output-expected/return_value

@@ -1 +1 @@
-0
+9

tools/tests/skdiff/test2/output-expected/return_value

@@ -1 +1 @@
-0
+9