[client] tests: add framebuffer rendering validation

Add an optional EGL framebuffer capture path for the test transport.
Capture the fully composed frame before presentation and retain its
surface format and HDR state for validation.

Add GoogleTest integration tests which run the production client
under a headless Weston compositor and compare every output pixel
against an independent reference implementation.

The matrix covers BGRA, RGBA, BGR32, RGB24, PQ RGBA10 and scRGB
RGBA16F with full, moving, overlapping, maximum-count, invalid, null
and zero-area damage rectangles. Validate HDR-to-SDR conversion and
native HDR output metadata when supported by the compositor.

Keep tests opt-in through ENABLE_RENDER_TESTS and document how to run
the software and native HDR tiers.

.github/workflows/build.yml

@@ -236,3 +236,47 @@ jobs:
       run: |
         cd doc
         make dirhtml SPHINXOPTS='-b spelling -W' -j$(nproc)
+
+  render-tests:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: recursive
+
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install \
+            weston libgtest-dev libgl1-mesa-dri \
+            libdw-dev libunwind-dev \
+            libspice-protocol-dev nettle-dev \
+            libgl-dev libgles-dev \
+            libx11-dev libxss-dev libxi-dev libxinerama-dev \
+            libxcursor-dev libxpresent-dev \
+            libwayland-dev libxkbcommon-dev libfontconfig-dev \
+            libsamplerate0-dev libpipewire-0.3-dev libpulse-dev
+
+      - name: Configure
+        run: |
+          cmake -S client -B client/build-render-tests \
+            -DOPTIMIZE_FOR_NATIVE=OFF \
+            -DENABLE_LIBDECOR=OFF \
+            -DENABLE_TEST_TRANSPORT=ON \
+            -DENABLE_RENDER_TESTS=ON
+
+      - name: Build
+        run: cmake --build client/build-render-tests --parallel
+
+      - name: Run rendering matrix
+        run: |
+          ctest --test-dir client/build-render-tests \
+            --output-on-failure \
+            -R looking-glass-render-tests
+
+      - name: Upload failed captures
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: render-test-failures
+          path: /tmp/lg-render-case.*