Convert gsplat material to be chunk-based

[slimbuck]

This PR:

• unifies the compressed and uncompressed gaussian splat material
• shaders leverage preprocessor #include and #if to organise shader code
• supports custom chunk overrides

Chunk breakdown as follows:

• Vertex shader:
  • gsplat: implements main(), common to compressed and uncompressed
  • gsplatCommon: common functionality like transforming gaussians 3d->2d and evaluating SH
  • gsplatData, gsplatSH, gsplatColor: uncompressed data access
  • gsplatCompressedData, gsplatCompressedSH: compressed data access
  • gsplatOutput: helper function for converting to target gamma space efficiently
• Fragment shader:
  • gsplat: common to compressed and uncompressed

[willeastcott]

How does this play with render passes? Don't render passes apply a tonemap as a post effect? Also, any reason not to apply fog?

[slimbuck]

Currently only standard-material supports fog, but I could add this to gsplat shader.

[willeastcott]

Would be nice to support it. But I'm happy for that to be a follow up PR. We're still calling it out as a limitation in the docs after all: https://developer.playcanvas.com/user-manual/graphics/gaussian-splatting/#limitations

[mvaligursky]

as long as the generator respects the related generation options (which it seems it is), it should work .. render passes pass the options they need

[mvaligursky]

I think it would be good to do the color animation in fragment shader as an example on how to do it .. it handles long splats better than per vertex.

[willeastcott]

With StandardMaterial, you would override a chunk via:

material.chunks.emissivePS = <shader_chunk_string>;

Does this PR make splat shader chunk overrides more consistent with this?

[slimbuck]

Does this PR make splat shader chunk overrides more consistent with this?

No. This PR primarily changes shader chunk layout and the way they're handled.

The material is a ShaderMaterial instance which does not support chunks. It might be worth implementing the material it as a GSplatMaterial instance instead, which would inherit from ShaderMaterial (presumably), with the additional chunk API.

However this also touches on GSplatComponent management, which I'm less sure of. @mvaligursky would have to comment, I think.

[mvaligursky]

fantastic improvement!