GLnemo2

I find the color stretching (feature #73) difficult to use compared with the ds9 one.

here are the differences I noticed:

- the colorbar stretching is very sensitive both on horizontal and vertical move.
in ds9, the range of the move corresponds to the full size of the screen (full screen, not the window).

- in ds9 the absolute mouse position maters whereas here it is relative
the stretch seems to be computed from the current mouse position compared to the initial position at the start of stretching.
in ds9 it is the absolute position compared to the full screen, ie one click may change the stretching

- the vertical down move seems buggy: only the min cut is changed where it is expected that both the min and max should move symmetrically.
this is the case when you go up (enlarge) but not when you go down

- when a cut reach the min or max value, it should not be bounded so that the behavior does not change, ie a reverse move is expected to go back to the previous stretch. It means if the min cut go below zero or the max cut go beyond 255 you loose some colors.

- in ds9 the data values outside the cuts are shown with the lowest or highest color, here there are discarded.
actually in ds9, there are scale settings (log/linear/etc... scales with threshold values) and the colorbar stretching is independent (ie the scaling can discard data where as the color bar stretching not...). Here the stretching is attached to the thresholds.