load_gltf doesn't preserve node's postion values

[nrayamajhee]

I've been encountering this issue persistent across all the gltf files I've tried where multiple nodes of the gltf scene are centered on the origin ignoring their position defined in the gltf file. You can reproduce the bug with the code from repo example:

    let default = concat!(env!("CARGO_MANIFEST_DIR"), "/test_data/Plane/scene.gltf");
    window.scene.add(&gltf);

    let mut mixer = three::animation::Mixer::new();
    for clip in gltf.clips {
        mixer.action(clip);
    }

I got the gltf from: https://sketchfab.com/models/f312ec9f87794bdd83630a3bc694d8ea#download

As you can see the propellers are not where they are supposed to be:

[kvark]

Pinging @alteous just in case