Octree: Add layers property for filtering objects.

[RemusMar]

This is a very useful feature (especially in games), it's 100% backwards compatible with the exisiting users logic and I thought to share it.
Long story short: many times you need to ignore a list of models/meshes when performing collisions or raycasting.
Think about the vegetation in a 3D world for example.
And here is this feature in action: https://necromanthus.com/Test/html5/SMC_Octree.html

[Mugen87]

Would you be okay if we implement this differently via layers similar to Raycaster? Meaning:

• Adding a new layers property to Octree.
• Make a layer test in fromGraphNode().

if ( this.layers.test( object.layers ) ) {

    // add to octree

}

If you shift objects to different layers, they should be excluded from the addition.

[RemusMar]

Hi Michael,
Sure, but I thought the current solution is the easiest and the fastest.

Also, I hope you noticed the "per material name" instead of "per mesh name" approach.
I think the "per material name" is the best solution, at least for 2 reasons:

1. think about 100 bushes sharing the same material.
   You want to add a single entry in that array, not 100
2. think about trees: you want collisions for the tree bark, but not for the upper part.

[Mugen87]

Yeah, that makes sense. But if the class works with layers, the app can use whatever criteria to configure the layers setting.

[RemusMar]

OK.
I've added Layers to Octree and updated the sample file: https://necromanthus.com/Test/html5/SMC_Octree.html
Same good result with only difference that the filtering is executed on the "client side" now:

	if (["palm1","palm2","palm3","bush1","bush2","bush3"].includes(child.material.name) ) {
		child.layers.set( 1 );
	}
// ...
camera.layers.enable( 1 );