TrackMate_: code style changes.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: add a drawing depth display setting.

Two settings allow to limit the drawing depth in the
HyperstackDisplayer. If an object is too far from the currently
displayed z-slice, it can be skipped when drawing the overlay.
Useful for dense annotations in thick images.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>