MeshBVHHelper: Update BVH Helper for instances and batching

[gkjohnson]

Fix #701

Adds MeshBVHHelper.objectIndex to support displaying a BVH for an individual BVH in an InstancedMesh or BatchedMesh.

cc @agargaro

[agargaro]

After you merge this PR, I can update BatchedMesh example to show it.

[gkjohnson]

That would be great! Thanks.

[gkjohnson]

I'm trying to decide what needs to be added to BatchedMesh to support what we're doing here with official, stable APIs. Use _drawInfo isn't great in the long term since it's intended to be internal. I suppose it could be made public? Or we could add some functions:

batchedMesh.getInstanceVisible( instanceIndex ); // returns visible && active
batchedMesh.getGeometryIndex( instanceIndex ); // returns drawInfo.geometryIndex
batchedMesh.getGeometryRange( geometryIndex ); // returns geometry drawRange object

What do you think?

[agargaro]

For me it would be better to have the methods, easier to use.

Is this already present?
batchedMesh.getInstanceVisible( instanceIndex ); // returns visible && active
https://github.com/mrdoob/three.js/blob/master/src/objects/BatchedMesh.js#L821

[gkjohnson]

So it is! We should adjust the extension function to use it in this line, then:

three-mesh-bvh/src/utils/ExtensionUtilities.js

Line 52 in be976e6

if ( ! drawInfo[ i ].visible || ! drawInfo[ i ].active ) {

If you'd like to make a three.js PR with the other two functions that would be great otherwise I can do so when I get a chance.

[agargaro]

All right, I'll do it 😁