Allow volumes to propose new ray directions

[mattngc]

• Volume emitters currently evaluate surface emission before volume emission. This execution flow doesn't allow modeling of scattering phenomena. To allow simulations of scattering and opacity effects, the emission calculation workflow inside volume materials needs to be redesigned.
• One possible solution is to separate scattering and emission processes inside the volume material. On initial ray intersection, material calculates scattering events and proposes new ray directions. In this way, the material can change the ray trajectory by breaking it into smaller segments inside the material. Each ray segment inside the material can be added to a stack, until the ray exits a surface. After recursively following the ray path, execution returns to the scattering material and emission is evaluated for every trajectory segment in the stack.