Render compressed GS data

[slimbuck]

This is a followup PR to #6357.

Up until now when loading compressed gaussian splat PLY files, we have simply decompressed the data at load time and rendered the scene as usual.

This PR adds a render path to load and render the compressed data directly, cutting memory consumption by 4x for the raw splat data and speeding up rendering by a modest 4%.

For the following bicycle scene:

	Old	New
Render time	13.4ms	13.0ms
Total VRAM	208MB	123MB
Load Time	6.0s	1.8s

NOTE: load time above includes the examples browser startup.

This PR makes the following changes:

• adds a createIter function to GSplatData, which constructs an iterator for accessing gaussian splat data. the iterator works the same whether the underlying data is compressed or not.
• makes use of the new iterator in the rest of GSplatData, making most code smaller and simpler.
• adds compressed versions of gsplat and gsplat-material for rendering compressed data directly.
• adds asset.data options which control whether to decompress the data at load time (useful for tools like super-splat) and also whether to reorder data for better GPU render performance.
• speed up gs bounding box calculation by using splat bounding sphere (i.e. overestimation) and rename the exact bound calculation calcAabbExact.

Todo:

• it may be possible to speed up rendering by early-out based on chunk bound (to be investigated)
• we might prefer to optimise the compressed format:
  • store v1v2 instead of rotation/scale
  • store rotation/scale/v1v2 in halfs instead of floats

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Name	Status	Preview	Comments	Updated (UTC)
engine	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 20, 2024 9:52am

[mvaligursky]

There is so much duplication between this and non-compressed material. I think we should unify them, and use ifdef inside shader to pick the right bits.

[slimbuck]

Yes there is some duplication, but I'd like to keep the two materials separate until the optimisations are final.

[willeastcott]

🔥🔥🔥