Fix REPLACE styling for glTF without pbrMetallicRoughness object

[lilleyse]

We noticed that Cesium3DTileColorBlendMode.REPLACE wasn't working for certain tilesets. Specifically, if a glTF material omitted the pbrMetallicRoughness object then the style would not get applied.

Why....? The code uses a special _3DTILESDIFFUSE semantic to figure out which attribute or uniform should be replaced with the feature color. Usually it attaches itself to the vertex color attribute, base color texture, or base color factor, but in this case none of those could be found.

The fix was to add a uniform for the default baseColorFactor value. The semantic gets added to that and everything works again.

As a side note, this bug doesn't seem to affect ModelExperimental which handles things very differently.

[cesium-concierge]

Thanks for the pull request @lilleyse!

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Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[ptrgags]

Looks good, thanks @lilleyse!