[glTF] Use a default material when none is specified

[footballhead]

Using scene::GltfLoader, some models in glTF-Sample-Assets fail to load because a material is not provided.

From 3.9.6:

The default material, used when a mesh does not specify a material, is defined to be a material with no properties specified. All the default values of material apply.

This would allow us to load more models like:

• AnimatedTriangle
• BoxVertexColors
• Cameras