Solari: Misc improvements

[JMS55]

• Search for a spatial neighbor up to 5 times for ReSTIR instead of just once.
• Bump RAY_T_MIN to 0.001. Prevents artifacts and doesn't really cause more self intersections.
• Re-calculate world cache cell_size after jittering the query position to prevent creating extra cells at the wrong LOD for a given position.
• Prevent the unlikely event of something checksumming to 0, which is reserved for empty cells.