Default to ignoreNothing() for image diff highlight layer

Change-Id: I017deb3be9a9423ab0a3449db180b1149363a6ce
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-image-viewer/gr-image-viewer.ts b/polygerrit-ui/app/elements/diff/gr-diff-image-viewer/gr-image-viewer.ts
index 1c0dd6b..dbad8ff 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-image-viewer/gr-image-viewer.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-image-viewer/gr-image-viewer.ts
@@ -658,6 +658,11 @@
     window
       .resemble(this.baseUrl)
       .compareTo(this.revisionUrl)
+      // By default Resemble.js applies some color / alpha tolerance as well as
+      // min / max brightness beyond which to ignore changes. Until we have
+      // controls to let the user affect these options, always highlight all
+      // changed pixels.
+      .ignoreNothing()
       .onComplete(result => {
         this.diffHighlightSrc = result.getImageDataUrl();
       });