workflows/lint: add clang-format on changed files

The entire codebase is not ready to be clang-formatted and probably
never will be, but we can at least check if the changes in new pull
requests follow our coding style.

.github/workflows/lint.yml

@@ -39,3 +39,24 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: chartboost/ruff-action@v1
+
+  clang-format-lint:
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
+    container:
+      image: alpine:3.20
+    # container:
+    #   image: "registry.opensuse.org/home/mia/images/images/mpv-ci:stable-deps"
+    steps:
+      - run: |
+          apk update
+          apk add clang18-extra-tools
+
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 50
+      - name: Check Formatting
+        shell: bash
+        run: |
+          git clang-format ${{ github.event.pull_request.base.sha }} --diff
+          echo $?