#15 glow effect

[Arnaud SINTES]

'glow' effect (Photoshop, seen in Lasse Hoile Steven Wilson's clips)

basically it consists to :
- duplicate the input image
- apply a hi_pass filter to it ('hi_pass_threshold' param)
- apply a Gaussian Blur to it ('blur_radius' param)
- flatten on the input image with a "screen" mode (simple addition?) ('merge_opacity' param)