"A.2 Reservoir Sampling" of PBR Book V4

[Wyman 2023] Chris Wyman, Markus Kettunen, Daqi Lin, Benedikt Bitterli, Cem Yuksel, Wojciech Jarosz, Pawel Kozlowski, Giovanni Francesco. "A Gentle Introduction to ReSTIR: Path Reuse in Real-time." SIGGRAPH 2023.