This commit allows Bevy to bind 16 lightmaps at a time, if the current
platform supports bindless textures. Naturally, if bindless textures
aren't supported, Bevy falls back to binding only a single lightmap at a
time. As lightmaps are usually heavily atlased, I doubt many scenes will
use more than 16 lightmap textures.
This has little performance impact now, but it's desirable for us to
reap the benefits of multidraw and bindless textures on scenes that use
lightmaps. Otherwise, we might have to break batches in order to switch
those lightmaps.
Additionally, this PR slightly reduces the cost of binning because it
makes the lightmap index in `Opaque3dBinKey` 32 bits instead of an
`AssetId`.
## Migration Guide
* The `Opaque3dBinKey::lightmap_image` field is now
`Opaque3dBinKey::lightmap_slab`, which is a lightweight identifier for
an entire binding array of lightmaps.
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