Add VSync graphics option, off by default

This is if you want delta-timesteps to go as quickly as possible. Also
it seems like on Windows this only has an effect in exclusive fullscreen
mode.

desktop_version/src/Graphics.h

@@ -324,6 +324,9 @@ public:
 	bool kludgeswnlinewidth;
 
 	Uint32 crewcolourreal(int t);
+
+	bool vsync;
+	void processVsync();
 };
 
 extern Graphics graphics;