BatchedMesh: Fix setInstanceCount().

[andreas-hilti]

Fixed #31457

Description

setInstanceCount tries to shrink availableInstanceIds as much as it can, by trying to remove ids at the end.
However, the condition was wrong, given that the largest id is instanceInfo.length - 1, not instanceInfo.length.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.04 78.86	338.04 78.86	+0 B +0 B
WebGPU	559.11 154.78	559.11 154.78	+0 B +0 B
WebGPU Nodes	558.03 154.56	558.03 154.56	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	469.29 113.56	469.29 113.56	+0 B +0 B
WebGPU	634.4 171.81	634.4 171.81	+0 B +0 B
WebGPU Nodes	589.53 161.13	589.53 161.13	+0 B +0 B

[gkjohnson]

Nice find! Thank you for the change. I see the PR is marked as draft. Are there any further changes you plan to make?

[andreas-hilti]

Nice find! Thank you for the change. I see the PR is marked as draft. Are there any further changes you plan to make?

Thanks. No, but the tests were not yet clean.