This has no user visible impact yet since smoke volumes only support a fixed
set of attributes, but will become important with the new volume object.
For GPU shader compilation, volume grids are now handled separately from
image textures. They are somewhere between a vertex attribute and an image
texture, basically an attribute that is stored as a texture.
Differential Revision: https://developer.blender.org/D6952
This further separates requested attributes and textures from the actual
node graph, that can be retained after the graph has been compiled and
freed. It makes it easier to add volume grids as a native concept, which
sits somewhere between an attribute and a texture.
It also adds explicit link types for UDIM tile mapping, rather than
relying on fairly hidden logic.
