bevy_solari: RIS for Direct Lighting

[JMS55]

Objective

• Start the realtime direct lighting work for bevy solari

Solution

• Setup all the CPU-side code for the realtime lighting path (minus some parts for the temporal reuse I haven't written yet)
• Implement RIS with 32 samples to choose a good random light
• Implement a shading pass (called reuse and shade, although there's no reuse yet)
• Don't sample a disk for the directional light, just treat it as a single point. This is faster and not much worse quality.

Future

• Skip the shadow map and light visibility passes (since solari replaces them)
• Spatiotemporal reuse (ReSTIR DI)
• Denoiser (DLSS-RR)
• Light tile optimization for faster light selection
• Indirect lighting (ReSTIR GI)

Testing

• Run the solari example to see realtime
• Run the solari example with -- --pathtracer to see the existing pathtracer

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Showcase

TODO: Screenshot

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