BatchedMesh: add support for resizing instance count, geometry size

[gkjohnson]

Related issue: #29464

Description

Adds support for Batched setGeometrySize and setInstanceCount. If adequate extents of the necessary geometry and instance buffers remain unused then the buffers can be shrunk, as well. However due to buffer fragmentation after adding / removing geometry and instances to and from the BatchedMesh it may not be possible to shrink the buffer - eg if 1000 instances are added and then the first 999 are removed. We could handle this case if we were to re-map ids so the data in this buffers could be rearranged without breaking references but we've decided not to do that for complexity reasons, which I think is okay.

To help with that issue the add geometry and instance functions have been adjusted to use earlier ids.

Tested by resizing geometry and instance buffers in the batched mesh demo and ensuring that there were no visual changes.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	688.12 170.5	689.82 170.9	+1.71 kB +403 B
WebGPU	808.01 217.65	809.73 218.11	+1.72 kB +462 B
WebGPU Nodes	807.52 217.52	809.24 217.98	+1.72 kB +462 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.84 111.78	462.84 111.78	+0 B +0 B
WebGPU	535.76 144.59	535.76 144.59	+0 B +0 B
WebGPU Nodes	491.87 134.33	491.87 134.33	+0 B +0 B