Modifiers: export motion blur velocity through attribute

Previously fluid simulation and Alembic modifiers had a dedicated function
to query the velocity for motion blur. Now use a more generic system where
those modifiers output a velocity attribute.

Advantages:
* Geometry and particle nodes can output velocity through the same mechanism,
  or read the attribute coming from earlier modifiers.
* The velocity can be preserved through modifiers like subdivision surface or
  auto smooth.
* USD and Alembic previously only output velocity from fluid simulation, now
  they work with velocity from other sources too.
* Simplifies the code for renderers like Cycles and exporters like
  Alembic and USD.

This breaks compatibility:
* External renderers and exporters accessing these velocities through the
  Python API now need to use the attribute instead.
* Existing modifier node setups that create an attribute named "velocity"
  will render differently with motion blur.

Differential Revision: https://developer.blender.org/D12305

source/blender/blenkernel/intern/modifier.c

@@ -1473,7 +1473,6 @@ void BKE_modifier_blend_read_data(BlendDataReader *reader, ListBase *lb, Object
         fmd->domain->tex_velocity_y = NULL;
         fmd->domain->tex_velocity_z = NULL;
         fmd->domain->tex_wt = NULL;
-        fmd->domain->mesh_velocities = NULL;
         BLO_read_data_address(reader, &fmd->domain->coba);
 
         BLO_read_data_address(reader, &fmd->domain->effector_weights);