Tweaks to asset sharing to support Unreal implementation

[kring]

This is a PR into #926. It goes with CesiumGS/cesium-unreal#1527.

[lilleyse]

Couple questions since I have a similar enum/struct thing in another project and would like to use this eventually

• Should it be called VertexAttributeSemantics in case we add InstanceAttributeSemantics in the future?
• Is there the possibility of allowing custom semantics like _FEATURE_ID_n?

[kring]

Should it be called VertexAttributeSemantics in case we add InstanceAttributeSemantics in the future?

Good idea, I'll rename it.

Is there the possibility of allowing custom semantics like _FEATURE_ID_n?

Do you have an idea what that would look like? It's not obvious to me what value we could provide for custom semantics in a class like this, but I could be missing something.

[lilleyse]

Do you have an idea what that would look like? It's not obvious to me what value we could provide for custom semantics in a class like this, but I could be missing something.

I think it would look the same as TEXCOORD_n and others. But I could also be missing context from CesiumGS/cesium-unreal#1527

[kring]

Oh, ok, I don't have any problem with adding extra commonly-used attributes from extensions to this class. I thought you were suggesting we make it externally-extensible somehow.