Use 4-byte LightmapSlabIndex for batching instead of 16-byte AssetId<Image>

[superdump]

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.

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).

---

Migration Guide

• Changed: RenderLightmap::new() no longer takes an AssetId<Image> argument for the asset id of the lightmap image.

[pcwalton]

LGTM! BTW I think AssetId is actually larger than 16 bytes because it doesn't just contain a UUID, it also contains an enum tag.