BatchedMesh: Deprecate old instancing render paths.

[CodyJasonBennett]

Description

Deprecates the now-unreferenced instanced multidraw rendering paths from #28103, which was superseded by #28404, and does not implement a fallback for Firefox. Instancing is since emulated by appending instances into the multidraw arguments themselves and using indirection for sorting/visibility, which has a fallback implemented for Firefox.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	689.92 170.93	690.08 171	+158 B +66 B
WebGPU	815.35 219.78	815.72 219.94	+365 B +165 B
WebGPU Nodes	814.86 219.65	815.22 219.82	+365 B +172 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	462.94 111.81	463.1 111.88	+158 B +67 B
WebGPU	538.09 145.28	538.45 145.42	+365 B +140 B
WebGPU Nodes	494.2 135.15	494.57 135.28	+365 B +133 B

[RenaudRohlinger]

I heavily rely on the multiDrawArraysInstancedWEBGL API in my projects, and the only current way of accessing it is through the object._multiDrawInstances property. Could we reconsider removing this feature?
Preserving access to the full WebGL API within Three.js is always an advantage, and removing support for this functionality would reduce its versatility.
Keeping it available ensures greater flexibility for developers who need advanced instancing techniques like instance multidraw.

[CodyJasonBennett]

This is an unmaintained code path that does not have a fallback for Firefox. Are you aware your projects will not work there? What about the overdraw lack of sorting is creating? Frankly, I think this is about trying to avoid a migration and not in the interest of the project since nothing is unreachable in practice if you implement BatchedMesh yourself and use indirection the three.js API now requires (instancing itself uses indirection, but this is another discussion). If this really were a discussion about low-level API, I have fielded this area quite heavily to enable multi-level mesh streaming with custom renderers and would suggest render methods over private fields or shapes. If you really aren't sure how to accomplish instancing in a custom BatchedMesh, I'm happy to tailor an example, but I maintain the solution we now implement is a strict improvement, and you should be using it.