Tracking: Mention TODO about DNA to Libmv API conversion

Current code works, but is heavily relying on matched enum definition
in the Blender's DNA and Libmv's API, which is suboptimal and fragile.

source/blender/blenkernel/intern/tracking_region_tracker.c

@@ -185,6 +185,8 @@ void tracking_configure_tracker(const MovieTrackingTrack *track,
                                 float *mask,
                                 libmv_TrackRegionOptions *options)
 {
+  /* TODO(sergey): Use explicit conversion, so that options are decoupled between the Libmv library
+   * and enumerator values in DNA. */
   options->motion_model = track->motion_model;
 
   options->use_brute = ((track->algorithm_flag & TRACK_ALGORITHM_FLAG_USE_BRUTE) != 0);