New global illumination system

[h3r2tic]

This completely overhauls the global illumination system, making it more accurate and robust to difficult light transport, and support large worlds out of the box.

The fixed-size radiance grid is gone, and so is its cone sweeping GI approach. In its place, an algorithm based in ReSTIR GI is implemented, along with a novel meshless irradiance cache.

Reflection denoising also gains ReSTIR, providing considerably better quality in difficult cases.

Moar info here: https://github.com/EmbarkStudios/kajiya/blob/restir-meets-probes/docs/gi-overview.md

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Still quite a bit of work needs to happen before this can be merged -- primarily a bunch of cleanup and optimizations.

• Fix data flow in reflection ReSTIR (bent lobe PDFs)
• Use ReSTIR in the irradiance cache (for faster updates)
• Reduce overhead of diffuse ReSTIR path validation
• Add path validation to reflection ReSTIR
• Disable the world radiance cache while it's still too janky
• Slim down texture formats of new resources
• Move fat irradiance cache buffers to textures not critical; later
• Disable the now-unnecessary ircache debug pass
• Make sure that the no-RTX path still works
• Rename/move restir_settings.hlsl
• Clean up the temporal/temporal2 mess
• Update documentation, add some comments
  • GI overview
  • • Reduced version of the old slides, with GI bits removed later.