Color Balance curves not preserved

[davidh]

1. add the Color balance, pull the nod of any of the curves enough to make the effect exaggerated for better visibility, click OK
2. reopen the transformation -> the curves look reset though the effect stays unchanged
3. copy the image to a separate branch -> the color effect is is no longer there

This does not happen when probing the image is used.

Multicolor Balance works as expected.

[jsachs]

Fixed for next release.