Add Russian roulette to GridDensityMedium::Tr().
Add Russian roulette to GridDensityMedium::Tr().

When the transmittance gets low, we start to randomly terminate the
transmittance computation. Speeds up rendering of the smoke/sphere scenes
by roughly 10% and reduces the average number of steps through the grid
from ~26 to ~13 for those scenes.

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