Add VSync setting.

OpenRA.Game/Settings.cs

@@ -138,6 +138,9 @@ namespace OpenRA
 		[Desc("This can be set to Windowed, Fullscreen or PseudoFullscreen.")]
 		public WindowMode Mode = WindowMode.PseudoFullscreen;
 
+		[Desc("Enable VSync.")]
+		public bool VSync = true;
+
 		[Desc("Screen resolution in fullscreen mode.")]
 		public int2 FullscreenSize = new int2(0, 0);