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

Some fixes for the post processing. Fixes both D3D, & OGL, which happened during the last update.

Also (by user request) changed the post-processing directory from the pcsx2 root, to a folder named 'shaders', and changed the file name from ambiguous shader.fx to GSdx.fx

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 ^.^