Fix cycles error for heterogenous volumes, causing double step size to be used.
Fix cycles error for heterogenous volumes, causing double step size to be used.

This gives longer render times due to smaller step size, double it to get
something more like the previous behavior.

tangent-opensource/coreBlackbird,pyrochlore/cycles,tangent-opensource/coreBlackbird,pyrochlore/cycles,pyrochlore/cycles,tangent-opensource/coreBlackbird