Custom properties defined on objects are not accessible from the
attribute node when rendering a volume in Cycles. This is because
this case is not handled.
To handle it, added a primitive type for volumes in the kernel,
which is then used in the initialization of ShaderData and to
check whether an attribute lookup is for a volume.
`volume_attribute_float4` is also now checking the attribute
element type to dispatch to the right lookup function.
Reviewed By: #cycles, brecht
Maniphest Tasks: T87194
Differential Revision: https://developer.blender.org/D11728
a5ed075110