Add smarter tolerance checking in the planar tracker.

The planar tracker uses Ceres for the refinement stage. During
refinement, Ceres iteratively updates the parameters with the
latest best guess. If the change in the parameters falls below a
threshold, Ceres will abort successfully ("converged").

For the case of pure translation tracking, the parameters are
exactly the two pixel shifts (dx, dy), and measuring the change in
these parameters gives a meaningful termination criterion.
However, for all the other parameterizations like affine, where
the parameterization involves affine parameters that have no
physical interpretation, Ceres is left with no way to terminate
the solver early. With the existing code, often many iterations
are run long after Ceres has found a solution sufficiently
accurate for all tracking needs. No one needs tracking with
a quadrillionth of a pixel accuracy; that time is wasted.

This patch extends the existing iteration callback that is passed
in to Ceres to check if the pattern has fallen out of the search
window, to also check if the optimizer has made a tiny step. In
particular, if the maximum shift of any patch corner between two
successful optimizer steps is less than a threshold (currently
0.005 pixels), the track is declared successful and tracking
is terminated.

This leads to dramatic speed increases in some cases, with little
to no loss in track quality. This is especially apparent when
tracking patches with affine or perspective motion models. For
example, on some tracking cases I tried, the iterations Ceres took
went from 50 to 3.

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