Support compressed.ply spherical harmonics

[slimbuck]

This PR:

• adds support for loading and rendering compressed spherical harmonic data in compressed.ply files
• the details of the format will be described elsewhere with the release in SuperSplat, but in summary:
  • 3 palettes of spherical harmonics coefficients are stored per file (one for each sh band)
  • each splat stores 9 indices into this data (r,g,b indices for band 1, 2, 3), totalling 128bits
• this results in compressed files being roughly twice the size with 3 bands of spherical harmonics
• small type fix for specifying texture data

Notes:

• we are adding support here for 0 or 3 bands of spherical harmonics, but may add explicit support for 1 and 2 bands in future
• the palette data is stored in halfs for the space savings, but it might be preferable to use floats

[willeastcott]

What an achievement. Is it possible to put some examples numbers in the PR....along with info about expected quality loss.