Shaders don't mark any exposed properties as "Main Color" about unitygltf

[hybridherbst]

Thanks — the ShaderGraphs in UnityGltf were compatible back to 2019.x where that didn’t exist, but now that we support 2021.x+ it’s a good idea to add that.

Worth noting that

breaks any code that uses "someMaterial.color" without checking

is a bit of a misnomer, code that doesn’t check it is already broken, not every shader even has a main color.

[hybridherbst]

@pfcDorn we should also add to the docs that shader properties in UnityGltf always match their glTF spec names for ease of use