Script-Fu -> High Pass

This is an adjustable script,
high pass filters have a long and varied history, this one is a re-work of script by Rob Antonishen. His blog post describing the original filter is here:
http://www.silent9.com/blog/archives/152-High-Pass-Filter-Plugin.html

High Pass is often used as a sharpening tool, but it is a bit more at home in the quot;contrast" department. To ease its use as a sharpening tool, however, I include the "overlay" option in the settings dialog.

Here's a list of the different modes:

	Colour - I think this is the same as photoshop's high pass filter effect.
	Preserve DC - like colour, but adds the average image colour back in.
	Greyscale - this desaturates the layer before high pass filtering.
	Greyscale, Apply Chroma - as above, but blends it in with the source layer colours
	Redrobes - (named after the technique described by Redrobes)

;  This script should be placed 
;  in the user or system-wide script folder.
;
;  Windows Vista/7/8/10)
;  C:\Program Files\GIMP 2\share\gimp\2.0\scripts
;  or
;  C:\Users\YOUR-NAME\.gimp-2.8\scripts
; 
;  Windows XP
;  C:\Program Files\GIMP 2\share\gimp\2.0\scripts
;  or
;  C:\Documents and Settings\yourname\.gimp-2.8\scripts   
; 
;  Linux
;  /home/yourname/.gimp-2.8/scripts