This is reusing the generic system from 128eb6cbe9 which at that time
was only done for the mesh reader and is now done for the point reader
as well.
This allows for rendering with proper motion blur for alembic points
(which of course are still imported as meshes - were just lacking the
velocity attribute) when directly rendering as points in Cycles.
Came up in #109185 (where instancing is used - which is still not
supported, this patch is just for direct point rendering in Cycles).
Unsure about the status of https://archive.blender.org/developer/D11591
but until that lands, having velocities for points sounds useful enough
to support this now.
Philipp Oeser