restir_spatial

Borrows reservoirs between neighbouring cells, one thread per grid cell.

Neighbouring cells stand on the same wall lit by the same sources, so a cell that drew a poor candidate can take its neighbour's. Every merge is evaluated against the borrowing cell's own surface, so a sample that does not apply here carries no weight.

Reads the temporal grid and writes a separate one, so a cell's answer does not depend on which invocations happened to run first.