Initial automated testing

.gitea/workflows/make_png.yaml

@@ -0,0 +1,27 @@
+name: Make PNG
+run-name: ${{ gitea.actor }} is recording the PNG output.
+
+on: push
+
+jobs:
+  build-app:
+    runs-on: linux_amd64
+    steps:
+      - name: Checkout curr
+        uses: actions/checkout@v4
+        with:
+          path: curr
+      - name: Checkout prev
+        uses: actions/checkout@v4
+        with:
+          path: prev
+          fetch-depth: 2
+      - run: cd prev && git checkout HEAD^
+      - name: Configure CMake
+        run: cmake -S curr -B curr/build
+        run: cmake -S prev -B prev/build
+      - name: Generate PNG files
+        run: cd curr/build/tests && xvfb-run ./test-basic
+        run: cd prev/build/tests && xvfb-run ./test-basic
+      - name: Compare PNG files
+        run: diff curr/build/tests/image.png prev/build/tests/image.png