velocity is measured in pixels per frame. It is basically a coordinate
difference of track coordinate at current frame and previous one (no future
prediction happens).
It's not really most intuitive place for such a things, but historically the
node was called this way..
Track velocity could be used to face effects like motion blur bu piping it
to the vector blur node.
Reviewers: campbellbarton
Reviewed By: campbellbarton
Subscribers: hype, sebastian_k
Differential Revision: https://developer.blender.org/D1591
2339a84443