Limit output SH bands

[slimbuck]

This PR updates the exporter to respect the current VIEW OPTIONS "SH bands" setting to limit the number of bands written to output ply and compressed.ply files.

Code changes are as follows:

• place the splat data extraction and transform logic into SingleSplat class
• SingleSplat reads the data for a gaussian and transforms it ready for output as follows:
  • convert between the scene SH bands and chosen output SH bands (currently non-used bands are just dropped, but in future we hope to add SH baking as an option).
  • applies the palette transform
  • rotates spherical harmonics
  • applies color/tint adjustments
• update the output code to use SingleSplat exclusively for accessing scene data

Notes:

• SuperSplat currently pads spherical harmonics to 3 bands at load time even if there are only 1 or 2 loaded. This means loading a model with 1 band and then saving directly will add 2 more bands. This will be addressed in future.
• The selection of output bands should be an option on an export dialog. This will be added soon.