adjust color curves: limit the curve points to function only
Submitted by Maciej Pilichowski
Link to original bug (#615981)
Description
Currently when you edit control points, gimp temporarily allows to create non-function curve, and after that (because such curve cannot exist) it resets the points. It is tiresome when you fine tune the curve.
Let's say you would like to create a curve with two control points in such shape:
+--- | | --+
so far so good, but when you drag the lower point to the right you end up with
The wish: (by example) the lower point should not be possible to move farther to the right than the upper point, and the upper point no farther to the left than the lower point.
IOW: x boundaries of current point: <prev point x position,next point x position>
Version: 2.6.6