[glTF] Apply color attribute during rendering

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](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#metallic-roughness-material) 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