DRY dilateFull method.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Add a comment on RandomAccessible possible bug.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Dilation: add an optimization for BitType.

Depending on the strel size and true pixel density, speedup ranges from
2 to 20.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Add more dilation methods.

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