Disable shadows when viewing point cloud markers

[srmainwaring]

🦟 Bug fix

Summary

The markers displayed by the Point Cloud widget cast shadows, which may not be intended.

To reproduce.

• Run a point cloud publisher, for example by using the executable from gz-gui/examples/standalone/point_cloud.
• Open an empty world, add the Point Cloud widget, then display the data. Any example world will do provided it includes lighting and a ground plane.

Before

After

The change adds a default material to the marker message. This triggers the MarkerManager system to handle the material, and the SetMaterial method for markers in the render engine disables shadow casting.

There is also a small change to the CMakeLists.txt in the point cloud example to allow it to be built on macOS (Darwin).

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
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• [x] All tests passed (See test coverage)
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