Blender Git Loki

Fix T91922: Cycles artifacts with high volume nested level

Make volume stack allocated conditionally, potentially based on the
actual nested level of objects in the scene.

Currently the nested level is estimated by number of volume objects.
This is a non-expensive check which is probably enough in practice
to get almost perfect memory usage and performance.

The conditional allocation is a bit tricky.

For the CPU we declare and define maximum possible volume stack,
because there are only that many integrator states on the CPU.

On the GPU we declare outer SoA to have all volume stack elements,
but only allocate actually needed ones. The actually used volume
stack size is passed as a pre-processor, which seems to be easiest
and fastest for the GPU state copy.

There seems to be no speed regression in the demo files on RTX6000.

Note that scenes with high nested level of volume will now be slower
but correct.

Differential Revision: https://developer.blender.org/D12759