Added feather length selection for grassfirealpha
Added feather length selection for grassfirealpha

On request for Broxton. If you don't want to feather the whole entire image then alternatively you can use the -f option. Adding an -f 20 will mean grassfirealpha only feathers in 20 pixels from an edge.

AveRapina/visionworkbench,DougFirErickson/visionworkbench,DougFirErickson/visionworkbench,fengzhyuan/visionworkbench,fengzhyuan/visionworkbench,fengzhyuan/visionworkbench,fengzhyuan/visionworkbench,fengzhyuan/visionworkbench,DougFirErickson/visionworkbench,DougFirErickson/visionworkbench,AveRapina/visionworkbench,DougFirErickson/visionworkbench,fengzhyuan/visionworkbench,AveRapina/visionworkbench,DougFirErickson/visionworkbench,AveRapina/visionworkbench,fengzhyuan/visionworkbench,AveRapina/visionworkbench,AveRapina/visionworkbench