[glTF] Use a default sampler when none is specified

[footballhead]

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

From the spec 3.8.2:

When texture.sampler is undefined, a sampler with repeat wrapping (in both directions) and auto filtering MUST be used.

This would allow us to load more models like:

• WaterBottle
• BoomBox
• Avocado
• BarramundiFish
• Lantern