Fix vertex shader generation for draco-compressed RGB per-vertex color

[likangning93]

Fixes #7576.

Draco-compressed models with RGB per-vertex color were causing vertex-shader compilation failure because the existing per-vertex-color shader assumes RGBA (vec4) per-vertex color, but the decoding glsl injected decodes to RGB (vec3).

This PR detects when COLOR_0 is getting decoded to vec3 and wraps access in the vertex shader in a vec4.

There's also a pair of new spec models generated by running BoxVertexColors and a modified variant with vec3 vertex color through gltf-pipeline for Draco compression.

I tried using draco-compressed versions of the "Vertex Color Test" model, but the checkVertexColors test function wouldn't produce the same results, I suspect because of compression artifacts making a difference when rendering at 1x1.

[cesium-concierge]

Thanks for the pull request @likangning93!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[likangning93]

@lilleyse re-ran CI and now it's passing

[lilleyse]

Looks good, thanks!