idea: Manipulate InstancedMesh instances with child elements

[trusktr]

f.e.

<lume-instanced-mesh count="10">
  <lume-instance position="1 2 3"></lume-instance>
  <lume-instance position="4 5 6"></lume-instance>
</lume-instanced-mesh>

or

<lume-instanced-mesh count="10">
  <!-- These elements are not rendered, and they determine the positions, etc, of the instances. -->
  <lume-element3d slot="instance" position="1 2 3"></lume-element3d>
  <lume-element3d slot="instance" position="4 5 6"></lume-element3d>
  <slot name="more-instances" slot="instance"></slot>

  <!-- This is rendered like a regular child (because it isn't slotted), is not involved with the mesh instances -->
  <lume-gltf-model position="10 20 30"></lume-gltf-model>
</lume-instanced-mesh>