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- Removed TRACK_PROCESSED flag which is actually unneeded now.
- When libmv fails to track marker for next frame, add marker
for new frame with old position and set it up as Disabled.
- "Break" track path when markers sequence is "broken".
This means path wouldn't be drawed over frames, during which
racking object was lost.
- Update Properties Editor for SpaceClip during playback.
Track preview button should now update nicely.
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