Add distance/gsd option to defocus mask generation

[jnb]

Hello,

The defocus mask is a great addition to the 1.7 version, but I think there is still room for improvement. When dealing with close-range photogrammetry, some areas in the background could indeed still be in focus but lacking resolution in comparaison with the foreground, leading to visible differences in the orthomosaic. Adding a max distance and/or min gsd option on top of the defocus mask generation would be great in those cases.

Besides, could you detail how the defocus threshold works ? In some cases it seems to operate improperly, leading to in focus areas to be masked whereas out of focus ones are not.

Thanks