Track memory allocated for volume grids.
Track memory allocated for volume grids.

nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,nispaur/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3