R3F instancing API - with mixed instancing types

r3f instances.jsx

// Each InstanceTemplate becomes an InstancedMesh behind the scenes.
// In this example we'll end up with 2 draw calls.
// This proposal uses a string `id` to refer to the templates, but it
// could also possibly be done by reference?

const Example = () => {
  return <Instances>
    <InstanceTemplate id="instancedBox">
      <boxGeometry />
      <meshStandardMaterial />
    </InstanceTemplate>
    <InstanceTemplate id="instancedSphere">
      <sphereGeometry />
      <meshStandardMaterial />
    </InstanceTemplate>

    {[0, 1, 2, 3, 4, 5].map(x =>
      <Thing position={[x, 0, 0]} boxColor={...} />
    )}
  </Instances>
}

const Thing = (props) => {
  return <group position={props.position}>
    <Instance template="instancedBox" color={props.boxColor} ... />
    <Instance template="instancedSphere" ... />
  </group>
}