Allow generation of background extraction in selected area
Hi this is my first merge request in both gitlab and Siril, I hope I'm doing this right.
This patch adds the possibility of doing an automatic background extraction on a selection of the image instead of the full image (or selecting manually), as I found I was very often removing the sample points from the edge of the image, especially when I intend to crop the picture anyway.
It works by adding a selection parameter to the relevant function and working on a sub-image instead of the full image.
Aris