BatchedMesh: Add geometry groups support and multi material handling about three.js

[gkjohnson]

I believe this is effectively a duplicate of #27203.

A unique material can be used, and in addition

Only a single material can be used across all geometry in the BatchedMesh. The demo from #27170 (comment) just demonstrates how a material can be modified to support different surface properties by packing them into a texture.

BatchedMesh is designed to minimize draw calls so I don't support changing the implementation to support an arbitrary number of materials which results separate draw calls, defeating the purpose of the class.

As I mention in #27203 it may be possible to shift the separate geometry tracking to the geometry object and share that among BatchedMesh instances so different materials can be used. But this hasn't been completely thought though. With the current BatchedMesh implementation in WebGLRenderer it should be possible to implement a custom version of the class with some of the behavior your reference without further changes to core. I think a clear API design and prototype should be demonstrated before more work is considered for this.