It's not that trivial to support disapearing tracks in the sequence,
and since we're having ongoing 2D stabilization rework patch wouldn't
want to start doing rather bigger changes here now.
Let's just stick to legacy behavior for this release.
This reverts commits 91429d0, 543ce85
File tracking.c became rather huge and annoying to
maintain and it really contains several independent
areas of motrack pipeline.
Now we've got:
* tracking.c: general-purpose functions which are used
by blender, clip editor, RNA and so.
* tracking_detect.c: feature detection functions
(blender-side, logic is still in libmv).
* tracking_plane_tracker.c: blender-side 2D tracking logic.
* tracking_plane_tracker.c: plane track tracker.
* tracking_solver.c: functions for camera solving.
* tracking_stabilize.c: 2D stabilization functions.
* tracking_util.c: utility functions for all those files
and which shouldn't be public.
