[glTF] Apply color attribute during rendering

[footballhead]

While GltfLoader::LoadMeshData will load the COLOR_0 accessor, it is not used by the StandardMaterial shader. The glTF 2.0 spec section 3.9.2 says:

In addition to the material properties, if a primitive specifies a vertex color using the attribute semantic property COLOR_0, then this value acts as an additional linear multiplier to base color

This would allow us to correctly render some glTF-Sample-Assets:

• BoxVertexColor
• CompareBaseColor
• IridescentDishWithOlives
• PrimitiveModeNormalsTest
• RecursiveSkeleton
• SheenWoodLeatherSofa
• VertexColorTest