Change resemble errorType to flat

ResembleJS features a lot of different errorTypes for various image
comparison. However, none of them are perfect. Likewise, flat is not
perfect as the diff image will often come out crude, but it will result
in the least false positives for the users.

Change-Id: I5e7d61dab28607fb870cea5905b0e23db13af7cc
diff --git a/gr-resemble-diff-mode/gr-resemble-diff-mode.js b/gr-resemble-diff-mode/gr-resemble-diff-mode.js
index a9e1f00..1e94c06 100644
--- a/gr-resemble-diff-mode/gr-resemble-diff-mode.js
+++ b/gr-resemble-diff-mode/gr-resemble-diff-mode.js
@@ -15,7 +15,7 @@
   'use strict';
 
   const DEFAULT_SETTING = {
-    errorType: 'movement',
+    errorType: 'flat',
     largeImageThreshold: 1200,
   };