geometry smoother for all model elements

[trusktr]

For example ability to control smooth shading of loaded models (adjust normals to make it looks smooth):

<lume-gltf-model ... smooth-geometry="30"></lume-gltf-model>
<lume-fbx-model ... smooth-geometry="30"></lume-fbx-model>

or

<lume-gltf-model ...>
  <lume-smooth amount="30"></lume-smooth>
</lume-gltf-model>
<lume-fbx-model ...>
  <lume-smooth amount="30"></lume-smooth>
</lume-fbx-model>

The first is easier to associate with upcoming CSS style features, while the second is more friendly to composability with various runtime templating systems.