PostProcess effect feedback

[vchelaru]

• [x] Do not nest PostProcessing .cs files so deeply. Consider just putting it in a root Graphics or EffectWrappers
• [x] Document the swapchain to explain why it works the way it does
• [x] All post processing effects should have 2 methods - one that takes a source and destination (which set the RenderTarget[0]) and one which assumes the render target is already set, so the effect only applies
• [x] XML doc-ify the interface so users know why it exists