ENH: LucasKanade:d_subpixel_init could be passed to optimize_translations

[domengorjup]

When starting the optimization loop in methods.LucasKanade.calculate_displacements, the input current image subset G is selected at the last calculated point location, rounded to nearest integer position. This enables us to never have to interpolate the current image G, only the reference image is interpolated.

The last calculated (subpixel) position of the observed point is an even better initial estimate to our optimization loop. The remainder when rounding the result of the previous optimization could possibly be passed as d_subpixel_init to the optimize_translations method, and could decrease the number of optimization iterations to convergence.

The correct direction of this remainder should be checked when implementing this, as well as the new number of iterations until convergence.