Make custom loaders that give you a Lume tree instead of a Three.js tree

[trusktr]

F.e. after this change, a <lume-gltf-model> element would have a resulting ShadowRoot that has a Lume tree loaded from the GLTF model, instead of a plain Three.js tree.

Then we'd be able to do things like

const gltfEl = document.querySelector('lume-gltf-model')
const el = gltfEl.shadowRoot.querySelector('#some-node')
el.setAttribute('has', el.getAttribute('has') + ' physical-material')
el.color = 'blue'

And then, if we have #263, we can manipulate the elements in a better way not relying specifically on the Three.js nodes (making it easier to abstract the HTML interface from Three, and for example use PlayCanvas).

With a Lume tree loaded into a ShadowRoot, then once we support CSS styling we can also style those elements with CSS features like ::part().

• gltf loader
• fbx loader
• collada loader