Add Screen::destroy()

In order to have squeaky-clean memory management, we'll need to clean up
all the things that Screen allocates. This is the function to call to do
so.

desktop_version/src/Screen.h

@@ -9,6 +9,7 @@ class Screen
 {
 public:
 	void init(const ScreenSettings& settings);
+	void destroy();
 
 	void GetSettings(ScreenSettings* settings);