Support GZ_GUI_RENDER_ENGINE_GUI_API_BACKEND env fallback

[j-rivero]

🎉 New feature

Summary

When the backend is not selected via the explicit C++ option (_renderEngineGuiApiBackend == nullptr), fall back to the GZ_GUI_RENDER_ENGINE_GUI_API_BACKEND environment variable before defaulting to OpenGL.

This lets tools that launch 'gz gui -c ' without forwarding the backend CLI flag still select an alternative backend via the environment. OpenGL remains the default when neither the option nor the env var is set; no behaviour change for existing users.

Checklist

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• Added tests
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• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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