GSdx: change post-processing directory & some shader fixes.

Some fixes for the post processing, for both D3D, & OGL, that happened during the last update.

I also by request, changed the post-processing directory from the pcsx2 root, to a folder named 'shaders'.

This tested fine on my end. On both D3D and OGL. But if anyone else wants to give it a test, it would be nice ^.^