ci: implement screenshot diffing

.github/workflows/ci.yml

@@ -309,3 +309,35 @@ jobs:
           FOLDER: docs
           SINGLE_COMMIT: true
           CLEAN: true
+  diff-reftests:
+    runs-on: ubuntu-latest
+    name: Diff reftest screenshots
+    needs: browser-test
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v1
+        with:
+          node-version: 12
+      - name: Cache node modules
+        uses: actions/cache@v2
+        env:
+          cache-name: cache-node-modules
+        with:
+          # npm cache files are stored in `~/.npm` on Linux/macOS
+          path: ~/.npm
+          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-build-${{ env.cache-name }}-
+            ${{ runner.os }}-build-
+            ${{ runner.os }}-
+      - name: Npm install
+        run: npm ci
+      - name: Checkout current snapshots
+        run: git checkout origin/gh-pages results
+      - name: Download test results
+        uses: actions/download-artifact@v2
+        with:
+          name: reftest-results
+          path: tmp/reftests
+      - name: Run diff
+        run: npm run reftests-diff