Fix discrepency between GD and Imagick versions in terms of supported blending modes.

[manticorp]

Some photoshop modes are easily recreated in GD, but are very slow because it requires pixel-by-pixel manipulation - hence using Imagick when it's available. HOWEVER, finding efficient ways in Imagick of replicating some of these effects without pixel by pixel processing is another story.

Hence - gotta just find a way to replicate the missing effects when imagick is available.