Render tests infrastructure

[tritao]

Summary

Add a render-test harness (CoinRenderTests) plus initial specs/scenes/baselines to support render regression testing.

Why

The later renderer/GL backend changes are large and hard to validate by hand; having a repeatable render regression workflow early in the stack makes review and iteration much safer.

Changes

• Add tools/CoinRenderTests and hook it into the build.
• Add initial render specs and scenes under tests/render.
• Vendor minimal third-party helpers used by the tool (thirdparty/whereami, thirdparty/stb).
• Refresh lighting_spot baseline to match software-rendered CI output.
• Run render tests in GitHub Actions on Ubuntu (under Xvfb + Mesa software GL).
• Note: small prerequisites are split into earlier PRs.

Testing

• Note: render-test tooling is opt-in (set -DRENDER_TESTS_ENABLED=ON; default is OFF).
• Matches CI (headless, software GL):
  • cmake -S . -B bld-rt-ci -G Ninja -DCMAKE_BUILD_TYPE=Release -DCOIN_BUILD_TESTS=OFF -DCOIN_BUILD_EXAMPLES=OFF -DRENDER_TESTS_ENABLED=ON
  • cmake --build bld-rt-ci -j$(nproc)
  • xvfb-run -a env LIBGL_ALWAYS_SOFTWARE=1 ctest --test-dir bld-rt-ci -R '^render_smoke$' --output-on-failure (passes)
  • xvfb-run -a env LIBGL_ALWAYS_SOFTWARE=1 ctest --test-dir bld-rt-ci -R '^render_gl$' --output-on-failure (passes 12/12)

Commits

• da4a66ca66: Add render tests infrastructure.

[tritao]

@VolkerEnderlein I sent this as a draft so you can check and provide some feedback about this.

My plans is to add a new modern rendering backend to Coin in the future, and this provides some testing infrastructure so we can make sure the new backend matches the existing one.

If you don't agree with this or think a different design is warranted, please let me know and I'll try to make those changes.

[VolkerEnderlein]

That really looks impressive. I am happy to accept this PR if it is ready to merge. Thanks for your ongoing work to push forward coin development.