Historically tracks and reconstruction for motion tracking camera
object were stored in the motion tracking structure. This is because
the data structures pre-dates object tracking support, and it was
never changed to preserve compatibility.
Now the compatibility code supports more tricks and allows to change
the ownership without breaking any compatibility. This is what this
change does: it moves tracks from motion tracking structure to the
motion tracking camera object, and does it in a way that no
compatibility is broken.
One of the side-effects of this change is that the active track is
now stored on motion tracking object level, which allows to change
active motion tracking object without loosing active track. Other
than that there are no expected user-level changes.
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