Implement multi-pass postprocessing shader.

This allows attaching multiple shader programs, processing the entire
screen as texture. Chaining shaders works, each shader will use
the output of the previous shader as input.

BBGE/AfterEffect.h

@@ -91,6 +91,7 @@ public:
 	void renderGridPoints();
 
 	void loadShaders();
+	void deleteShaders();
 
 	void unloadDevice();
 	void reloadDevice();