Only count volume objects after shader optimization.

Allows to discard objects which don't have effective volume
BSDF connected to the shader output (i.e. constant folded,
or non-volume BSDF used by mistake).

Solves memory regression reported in T92014.

There is still possibility to improve memory even further
for cases when there are a lot of non-intersecting volume
objects, but that requires a deeper refactor of update
process. Will happen as a followup development.

Depends on D12795.