Steam Frame build, packaging and deployment scripts; render test

- build_scripts/steamframe/: Debian trixie (Qt 6.8) containers for the
  aarch64 cross build, native build and tests; toolchain; scripts to build,
  test (Vulkan validation layers), package (system Qt, ~3 MB) and deploy over
  SSH. Adapted from the vrdashboard template.
- test/render/: render test for the OpenGL and Vulkan backends, driven like
  OverlayController (option OVRAS_BUILD_TESTS).
- RPATH only $ORIGIN, without the build machine's link directories.
- Ignore dist/.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

build_scripts/steamframe/test.Dockerfile

@@ -0,0 +1,10 @@
+# Test environment on top of native.Dockerfile: Mesa Vulkan/GL drivers (incl.
+# the lavapipe software rasterizer), the Khronos validation layers and Xvfb.
+# See container-test.sh. Adapted from the vrdashboard template.
+FROM ovras-steamframe-native
+
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends \
+        mesa-vulkan-drivers vulkan-validationlayers vulkan-tools libgl1-mesa-dri \
+        xvfb xauth \
+ && rm -rf /var/lib/apt/lists/*