MaMuT_: protect against out of bounds when fetching thumbnails

Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
TrackMate_: properly handle coumpounds delete commands

When removing several spots and edges from the model at once - typically
a whole track -, the track model still process each removal at once.
Therefore we shall take no hurry in removing some ids too early.
There is still a pb wth this, though, that needs further investigating.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
MaMuT_: the semi-automated tracker exploits different mipmap levels

The optimal mipmap level is computed from the initial spot radius. A
source image correctly downsampled is then extracted from the source to
speed up calculations.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>