VOXEL: add pathtracer and PBR attributes

[mgerhardy]

Support should be added as a general module to be re-used in thumbnailer, voxconvert and voxedit at the same time.

The pathtracer yocto should be used: https://github.com/xelatihy/yocto-gl

https://xelatihy.github.io/yocto-gl/yocto/yocto_scene/

Also implement PBR attribute handling - see #267

https://pbr-book.org/