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>

CMakeLists.txt

@@ -283,8 +283,10 @@ if(NOT WIN32 AND NOT APPLE)
     set_property(TARGET AdvancedSettings PROPERTY OUTPUT_NAME AdvancedSettings_app)
     # libopenvr_api.so is copied next to the executable (see "openvrApiCopy");
     # $ORIGIN makes the loader look there, in the build tree and in packages.
+    # BUILD_WITH_INSTALL_RPATH: only $ORIGIN, without the build machine's
+    # link directories (the build tree is what gets packaged).
     set_target_properties(AdvancedSettings PROPERTIES
-        BUILD_RPATH "$ORIGIN"
+        BUILD_WITH_INSTALL_RPATH ON
         INSTALL_RPATH "$ORIGIN"
     )
 endif()
@@ -594,3 +596,12 @@ install(SCRIPT ${deploy_script})
     #        "${COPY_DEST_DIR}/AdvancedSettings.exe"
     #)
 endif ()
+
+# Tests (off by default). test/render checks the render backends
+# (docs/steamframe/DESIGN.md, "Tests"); build_scripts/steamframe/
+# container-test.sh runs them with the Vulkan validation layers.
+option(OVRAS_BUILD_TESTS "Build the tests" OFF)
+if (OVRAS_BUILD_TESTS)
+    enable_testing()
+    add_subdirectory(test/render)
+endif ()