Fix the order of application of color correction curves in the "Curves" filter

Now the order of transformations fits the order of other popular
applications, including Gimp:

1) Apply per-channel color curves
2) Apply composite RGB color curve, which changes all the channels
   at once
3) Apply the lightness curve in the end

CC:kimageshop@kde.org
BUG:324332
Fixes T2083