The AO baking procedure consists of the following steps:
1. Flatten the glTF hierarchy.
2. Generate a single 2D parameterization for the entire scene.
3. Embree Pass 1: Create G-Buffer using the above UVs as vert positions.
4. Embree Pass 2: Cast rays from the positions embedded in the G-Buffer.
The `gltf_baker` tool is not ready for general use but already
produces reasonable results for certain well-formed models.
