Better fix of edge and spot coloring.

We now generate the colors on the fly for spots, which solves the
problem we met with 3D viewer interactions. It has two other
consequeces:
- lower memory footprint (before that we were storing a Map<Spot, Color>
- maybe a price to pay on responsiveness - not noticable with my light
  tests.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>