wuffs: update, add jpeg decoding, add simple tests (#4250)

1. Update wuffs to v0.4.0-alpha.9
2. Add JPEG decoding
3. Add basic unit tests for image decoding
4. Add CI jobs to run wuffs unit tests.

.github/workflows/test.yml

@@ -513,3 +513,38 @@ jobs:
           useDaemon: false # sometimes fails on short jobs
       - name: typos check
         run: nix develop -c typos
+
+  test-pkg-linux:
+    strategy:
+      fail-fast: false
+      matrix:
+        pkg: ["wuffs"]
+    name: Test pkg/${{ matrix.pkg }}
+    runs-on: namespace-profile-ghostty-sm
+    needs: test
+    env:
+      ZIG_LOCAL_CACHE_DIR: /zig/local-cache
+      ZIG_GLOBAL_CACHE_DIR: /zig/global-cache
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup Cache
+        uses: namespacelabs/nscloud-cache-action@v1.2.0
+        with:
+          path: |
+            /nix
+            /zig
+
+      # Install Nix and use that to run our tests so our environment matches exactly.
+      - uses: cachix/install-nix-action@v30
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - uses: cachix/cachix-action@v15
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
+      - name: Test ${{ matrix.pkg }} Build
+        run: |
+          nix develop -c sh -c "cd pkg/${{ matrix.pkg }} ; zig build test"