defringe: first version for usermanual
defringe: change phrasing for usermanual a bit
699: explicitly shield dev->image_storage with a mutex

now that it's written from multiple threads.
defringe: add more informaion and change some parts of the usermanual
defringe usermanual: remove strength because it will also be removed in the iop because of too little effect and being mostly reproduceable with blending
Merge pull request #705 from bluesceada/defringe-usermanual

Defringe usermanual
defringe tuning: correct out of bounds check in fib_latt (just for safety, should never really happen at runtime)
defringe tuning: correct wrong *2.0 in local average and have a minmax of 0.1f
defringe tuning: remove strength slider + tune threshold & radius settings + picture for usermanual w/o strength slider