TrackMate_: control visibility at the spot level

VISIBILITY is now a spot feature, which 0 or less than 0 for invisible
spots.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: avoid unnecessary boxing an unboxing for spot features

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>