Improved color aberration auto-correction

I believe this branch is ready for integration.

Main changes :
* No loss of color saturation
* Smooth color shifts (no block discontinuities)
* Significantly more accurate measure to estimate color shifts
* The fitting was improved
* Restored OpenMP support
* Improved border reconstruction
* Clean code

It's activated on export, and in the GUI in zoomed mode by default.

I will try to convert the radial linear fit to a radial polynomial later on, and improve the generic fit as well. (This may require linking against the gnu scientific library.)

Anyway, I believe the current code is better than the legacy code in every aspect. So enjoy!

Minor: `free(NULL)` is perfectly fine, no need to check.
Also see dfa494e9fdaebc6fb8d23032654ed55c5502481d