Camera tracking integration

Camera tracking integration
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- Fixed discovered bug with selecting hidden marker areas with mouse.
- Refactor of context structure used for 2D tracking:
  * Tracker object is now per-track. This allows to use
    different pyramid levels for different tracks and
    now search area resizing shouldn't make tracking less
    accurate.
  * This is also be used for new optimized libmv API.
  * This allowed to use OpenMP to track different markers in
    separated threads.
    This allowed to reach quite cool speedup for multi-core CPU.