Sogs followup work

[slimbuck]

We recently added runtime support for loading and rendering SOGS (plas) data in #7510.

The following work is outstanding to match compressed.ply rendering performance:

• reorder data after download (for better rendering performance) into morton order. this requires:
  • generate morton order for gaussian centers
  • reorder the GPU data using this order
• construct chunk data similar to compressed.ply
  • this will allow histogram-based sorting

In addition, the SOGS data could do with more massaging:

• combine labels_l and labels_u
• combine sh0 and opacities
• investigate using avif and possibly squoosh webp -> lossless -> slight loss & max effort where appropriate