[glTF] Support meshes without positions

GltfLoader::LoadMeshData assumes that mesh primitives always provide the positions attribute. However, the gltf 2.0 spec section [3.7.2.1](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#meshes-overview) 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.