Allow "Edge" slider to go to -100 (currently can only go to -50)

AI sky masks often must be feathered entirely within the sky, without bleeding into the background below.  With the current Feather/Edge sliders, Feather can go to 100, but Edge can only go to -50.  Therefore, the practical maximum is Feather 50 and Edge -50, so that the feather remains within the sky.  The Edge slider should go to -100, so that the full feather can be applied within the sky.