[renderer][volumetric](up to 15 pts) Volumetric Rendering

# Volumetric Rendering

Volumetric rendering is critical to create visual effects such as colored glass fogs, clouds, jades, etc.

Implement volumetric absorption (**2 pts**)
Implement participating volumes (**3 pts**)
Implement volumetric lighting (**2 pts**)
Implement heterogeneous volumes (**4 pts**)
Accelerate heterogeneous volume sampling (**4 pts**)