* Space: volume density and step size in object or world space
* Step Size: override automatic step size
* Clipping: values below this are ignored for tighter volume bounds
The last two are Cycles only currently.
Ref T73201
Voxels are loaded directly from the OpenVDB grid. Rendering still only supports
dense grid, so memory usage is not great for sparse volumes, this is to be
addressed in the future.
Ref T73201
Rather than passing around void pointers, various Blender image sources now
subclass this. OIIO is also just another type of image loader.
Also fixes T67718: Cycles viewport render crash editing point density settings
