MLT: update progress more frequently for boothstrap samples
MLT: update progress more frequently for boothstrap samples

This is fine now with the ProgressReporter improvements.

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