Less data accessed and compared gives better batching performance.
# Objective
- Use a smaller id to represent the lightmap in batch data to enable a
faster implementation of draw streams.
- Improve batching performance for 3D sorted render phases.
## Solution
- 3D batching can use `LightmapSlabIndex` (a `NonMaxU32` which is 4
bytes) instead of the lightmap `AssetId<Image>` (an enum where the
largest variant is a 16-byte UUID) to discern the ability to batch.
## Testing
Tested main (yellow) vs this PR (red) on an M4 Max using the
`many_cubes` example with `WGPU_SETTINGS_PRIO=webgl2` to avoid
GPU-preprocessing, and modifying the materials in `many_cubes` to have
`AlphaMode::Blend` so that they would rely on the less efficient sorted
render phase batching.
<img width="1500" alt="Screenshot_2025-03-15_at_12 17 21"
src="https://github.com/user-attachments/assets/14709bd3-6d06-40fb-aa51-e1d2d606ebe3"
/>
A 44.75us or 7.5% reduction in median execution time of the batch and
prepare sorted render phase system for the `Transparent3d` phase
(handling 160k cubes).
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## Migration Guide
- Changed: `RenderLightmap::new()` no longer takes an `AssetId<Image>`
argument for the asset id of the lightmap image.
bc7416aa22