MSFT_lod: Clarify the behavior of the case where node level and material level LODs are specified

From Khronos Slack

The expected behavior of a case where both node level and material level LODs are specified is a bit unclear in `MSFT_lod` extension spec.

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

> If both node level and material level LODs are specified then the material level LODs only apply to the specific node LOD that they were defined for.

I would like to suggest to clarify the behavior especially it would be good if the spec explains with example.

And, do we really need a capability that the both are specified? In general lower LOD nodes have references to lower quality meshes and lower quality meshes can refer to lower quality materials. In this case, material LOD specification may not be needed because node LOD can cover it.

Start with extensions/2.0/Vendor/MSFT_lod and read the quoted rule for cases where node-level and material-level LODs coexist. Clarify the behavior with an example in the extension spec, and resolve whether supporting both levels is necessary based on the issue's stated relationship between lower-LOD nodes, meshes, and materials.

Written by the indexing model from the issue text.