SelectionBox: Add support for InstancedMesh.

[webzep]

Related issue: https://discourse.threejs.org/t/how-to-select-objects-with-a-selection-rectangle/4772/14

Description

SelectionBox.js does not give information about which instances in an InstanceMesh are in the frustum. This change adds a property called instanceDetection to the SelectionBox class.

When the searchChildInFrustum method is called, the instanceDetection property will update with information about which instances of an InstanceMesh are in the frustum.

An example of this.instanceDetection when the selection includes the instance at index 12 is below. The InstanceMesh uuid gets added to the object with a value of an array which includes all instanceIds that are in the frustum.

{
    "460A8174-2AD7-4F0B-96B1-3DD4CA57AE3A": [12]
}

[mrdoob]

Thanks!

[mrdoob]

Did some clean up... b88cb7f

@Steviebaa Isn't it possible to add the results to the collection array somehow?

[webzep]

@mrdoob Ideally, I think the results would look like the raycaster results where the array items have an object key with the mesh and an instanceId if it is InstanceMesh. Changing to that will no doubt break existing code though.

Sorry about the indentation... first PR 😆. Maybe a prettier config file could be good for the project?