Blender Git Loki

Compositor: Expose track velocity via the Track Position node

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