Function blender_mesh::flatten_exported_meshes

pub fn flatten_exported_meshes(
    meshes_by_filename: &MeshesByFilename
) -> Result<HashMap<&str, &BlenderMesh>, FlattenMeshError>

Convert MesheshByFilename into a HashMap<MeshName, &BlenderMesh> that flattens all of the meshes across all of the files into one HashMap.

This will error if there are two meshes with the same name across two or more files.