Blender Git Loki

Cycles X: more complete support for volumes

Brings back volume rendering support to what it was before on the GPU. That
means scattering, emissions and heterogeneous volumes with stepping. However
no decoupled or equiangular sampling yet.

Rather than separate homogeneous and heterogeneous code paths, there is only
heterogeneous now. This ensure shader evaluation is only compiled once. Some
optimizations may be possible for the homogeneous case, but that can be looked
at later.

Includes multiple commits, these are split in P2216. We can discuss how
best to review this next week.

Differential Revision: https://developer.blender.org/D11709