Added Screen interface

src/sdlf/Screen.hpp

@@ -0,0 +1,21 @@
+#pragma once
+
+#include "SDL.h"
+
+namespace sf
+{
+	class IScreen
+	{
+	public:
+
+	protected:
+		IScreen();
+
+		virtual void OnFocus() = 0;							// Called when the screen is being switched to
+		virtual void OnDefocus() = 0;						// Called when the screen is being switched from
+		virtual void OnUpdate(double frametime) = 0;		// Called every frame
+		virtual void OnEvent(const SDL_Event& event) = 0;	// Called after an event occurs
+	
+	private:
+	};
+}