This adds support to render PointCloud motion blur from a standard
"velocity" attribute.
This implementation is similar to that of the Mesh geometry, and
perhaps some code could be deduplicated through a more generic API.
`mesh_need_motion_attribute` was renamed `object_need_motion_attribute`
as it does not really require a mesh and moved to `util.h` so that
it can be shared.
This fixes T94622.
Reviewed By: brecht
Maniphest Tasks: T94622
Differential Revision: https://developer.blender.org/D13719
4c3f52e7dc