Make a freehand selection of a segment within one path

[Ofnuts]

Update: OK, found a way to avoid eating too much memory. On my test path and on a bad case, the picture is 15.3MB and jumps to 255MB with the current version, and to a mere 15.8 MB with the fix. It also runs a lot faster, because 1) Gimp has a lot less undo things to care about and 2) I found a API that disables the Path display update while the script runs.

Now to plug the fix in each of the various OPE operations that require it.