Add the VideoCommon PostProcessing class.

This class loads all the common PP shader configuration options and passes those options through to a inherited class that OpenGL or D3D will have.
Makes it so all the common code for PP shaders is in VideoCommon instead of duplicating the code across each backend.

Source/Core/VideoCommon/RenderBase.cpp

@@ -59,6 +59,8 @@ int Renderer::s_target_height;
 int Renderer::s_backbuffer_width;
 int Renderer::s_backbuffer_height;
 
+PostProcessingShaderImplementation* Renderer::m_post_processor;
+
 TargetRectangle Renderer::target_rc;
 
 int Renderer::s_LastEFBScale;