Use correct default for glTF occlusion texture.

[kring]

The CesiumGlTFFunction material function has an incorrect default for the occlusionTexture. As a result, a glTF model without an explicit occlusion texture (which is most of them) will have an Ambient Occlusion value of 0.0, rather than 1.0 as expected. Under certain lighting setups, this can make shadows cast onto the model extremely dark (basically pure black).

This PR fixes it by changing the default texture to one that has the value 1.0 in the red channel. Specifically, from this (NoMetallicRoughnessTexture):

To this (NoColorTexture):

(Thanks @AlbanBERGERET-Epic for reporting this!)

[cesium-concierge]

Thanks for the pull request @kring!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium for Unreal Samples works.

[j9liu]

@kring did you save the material with Unreal Engine 5.1? I'm viewing this in UE 5.0 and I'm getting this message:

 Error: Package C:/Dev/cesium-unreal-samples/Plugins/cesium-unreal/Content/Materials/MaterialFunctions/CesiumGlTFFunction.uasset is too new. Engine Version: 1004  Package Version: 1008

It's preventing anything in the scene from rendering with the material. Let me see if I can recreate the change in Unreal 5...

[kring]

Oh yeah I probably did @j9liu. Sorry about that, and thanks for fixing it!

[j9liu]

Thanks @kring !