Add the PostProcessing class object to RenderBase in VideoCommon.
Backends will initialize this variable with their own inherited PostProcessing class object.
Change OpenGL's post processing to use the new VideoCommon PP object.

Let's OpenGL's PostProcessing namespace be changed to a class inheriting from VideoCommon's PostProcessing class.
Change all the post processing shaders to the new system.
Removes the README.txt file in favour of a new wiki page I'm going to generate.
Add the configuration dialog for post processing configuration options.
Only enables the config button when the shader options available to it