When checking the Cage Transform operation code, I saw some ifdef meant
to be switched to when bug 645810 is fixed. It also says in a comment:
> /* When Gegl bug #645810 will be solved,
>    this should be a good optimisation */
Checking said bug, it appears it has been fixed since 2012!
I also fixed a bit the parameters in gegl_buffer_sample() call since it
seems the function signature has changed quite a bit.