Detect unlit/lit material props when loading GLTFs and set their material accordingly

[pravusjif]

Today every GLTF is loaded with a PBR material.

We should be able to detect either the usage or the missing usage of PBR properties in the original GLTF Material and configure the Decentraland material+shader accordingly.

Key point: ALPHA MASK is not supported on PBR materials anymore, if creators want to keep using ALPHA MASK on GLTFs, they have to configure the GLTF Material to represent an Unlit material.

Raw GLTF Material generation: https://github.com/decentraland/unity-shared-dependencies/blob/b1a299af370b59d141cba3ad49149ea209bc0be1/Runtime/GLTFastWrappers/DecentralandMaterialGenerator.cs#L35