[glTF] Support meshes without positions

[footballhead]

GltfLoader::LoadMeshData assumes that mesh primitives always provide the positions attribute. However, the gltf 2.0 spec section 3.7.2.1 says:

When positions are not specified, client implementations SHOULD skip primitive’s rendering unless its positions are provided by other means (e.g., by an extension). This applies to both indexed and non-indexed geometry.