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
128eb6cbe9