MLT: better chunk size for bootstrap paths
MLT: better chunk size for bootstrap paths

Split into smaller chunks for better thread utilization when we have a small number of bootstrap paths.

nyue/pbrt-v3,mwkm/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,bssrdf/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,ambakshi/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,mwkm/pbrt-v3,bssrdf/pbrt-v3,ambakshi/pbrt-v3,nispaur/pbrt-v3,shihchinw/pbrt-v3,nyue/pbrt-v3,bssrdf/pbrt-v3,nispaur/pbrt-v3,ambakshi/pbrt-v3