MSFT_lod: Clarify whether recursive LOD is allowed or not

[takahirox]

https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Vendor/MSFT_lod

In MSFT_lod spec, it's unclear whether recursive LODs are allowed or not. For example

"nodes": [
    {
        "name": "High_LOD",
        "mesh": 0,
        "extensions": {
            "MSFT_lod": {
                "ids": [
                    1,
                    2
                ]
            }
        }
    },
    {
        "name": "Medium_LOD",
        "mesh": 1,
        "extensions": {
            "MSFT_lod": {
                "ids": [
                    3,
                    4
                ]
            }
        }
    },
    {
        "name": "Low_LOD",
        "mesh": 2
    },
    ...
]

node: 0 has the references to node: 1 and node: 2 as LODs and node: 1 has the references to node: 3 and node: 4 as LODs. How should the LODs for node: 0 be?

Probably allowing the recursive LODs makes the engines complex so we may have two options.

1. glTF data including recursive LOD is invalid.
2. The engines should apply the top-level LOD specification and ignore the recursive LOD specifications.

Perhaps 2. is good? In the example below, processing node: 0 will ignore the LOD specification in node: 1 so node: 0 will be High-Middle-Low LODs. And processing node: 1 will apply the LOD specification in node: 1 so node: 1 will be Middle-Low LODs.

"nodes": [
    {
        "name": "High_LOD",
        "mesh": 0,
        "extensions": {
            "MSFT_lod": {
                "ids": [
                    1,
                    2
                ]
            }
        }
    },
    {
        "name": "Medium_LOD",
        "mesh": 1,
        "extensions": {
            "MSFT_lod": {
                "ids": [
                    2
                ]
            }
        }
    },
    {
        "name": "Low_LOD",
        "mesh": 2
    },
    ...
]

[bghgary]

@najadojo Ditto for this. What happens for glTF-Toolkit and Windows Mixed Reality?