Tweak solari world cache

[JMS55]

• Make the world cache transition LODs faster. On big scenes like bistro, we had way too many small cells before.
• Limit world cache GI rays to 4m, we save a ton of time and improve stability by not sampling super far away cells.
• Switch world cache active cell update workgroup size from 1024 -> 64. Using large workgroup sizes for RT work kills occupancy.

World cache sample_radiance time goes from 1.42ms to 0.09ms.

[HugoPeters1024]

I can attest from experience that 1024 is indeed far too large a work group size for raytracing workloads. Although the optimal is also hardware dependent, 64 is a reasonable default