This commit adds a Volume Cube primitive node. It outputs a volume that
contains a single "density" float grid. The density per voxel can be
controlled with a field that depends on the voxel position (using the
existing Position node). Other field inputs are not supported.
The density field is evaluated on every voxel.
Possible future improvements are listed in D15198.
Differential Revision: https://developer.blender.org/D15198
838c4a97f1