renderdiff: add viewer for image differences (#8768)

- Modify the compare script to output more details of a
   comparison. This will include the source/golden directory,
   the comparison directory (the new renderings), and a file
   path to difference images if the golden does not match
   the rendered image.
 - The image_diff script can now output a TIFF that is the
   difference of two input TIFFs.
 - Add a viewer for examining the differences between rendered
   output and golden images.
 - The viewer consists of a simple server of web API endpoints
   for querying difference results (along with rendered images
   in TIFF).
 - And a web-based (html + lit-element) UI for looking at the
   rendered images and differences.

test/renderdiff/src/render.py

@@ -102,7 +102,7 @@ if __name__ == "__main__":
                         vk_icd=args.vk_icd)
 
   with open(f'{output_dir}/render_results.json', 'w') as f:
-    f.write(json.dumps(results))
+    f.write(json.dumps(results, indent=2))
 
   shutil.copy2(args.test, f'{output_dir}/test.json')