ProgressReporter: use int64_t to track work done.
ProgressReporter: use int64_t to track work done.

(Missed this when switching to an int64 for totalWork.)

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