BatchedMesh: add getColorAt and setColorAt

[Kikedao]

Following the discussion at #28151, I'm creating this PR to add per-instance get/setColorAt() for BatchedMesh.

I took special care in doing it in a way similar to how setColorAt() is implemented for InstancedMesh so no additional computation happens in the shaders in case no color is ever set.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
676.8 kB (167.9 kB)	678.7 kB (168.2 kB)	+1.88 kB

🌳 Bundle size after tree-shaking

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

Filesize dev	Filesize PR	Diff
455.9 kB (110.1 kB)	456.9 kB (110.3 kB)	+1.01 kB

[gkjohnson]

It might be worth updating the batched mesh example to show color rendering, as well.

[Kikedao]

It might be worth updating the batched mesh example to show color rendering, as well.

For InstancedMesh we have several examples, one with a normal material ("instancing / performance") and one where the color setting and raycasting is showcased ("instancing / raycast").
Do you want me to create a new example similar to the raycasting one for BatchedMesh to not mess with the existing one ("mesh / batch")?

[gkjohnson]

Do you want me to create a new example similar to the raycasting one for BatchedMesh to not mess with the existing one ("mesh / batch")?

cc @Mugen87 do you think an example update is needed at all?

[Mugen87]

Not necessarily.

[gkjohnson]

Not necessarily.

Sounds good to me, then. Once the other couple comments are addressed I think this is ready