Config: Don't save --fullscreen unless changed.

This makes --fullscreen and --windowed temporary by using a separate
setting.  If the setting is changed explicitly, it begins saving it again.

Windows/MainWindowMenu.cpp

@@ -955,7 +955,7 @@ namespace MainWindow {
 			break;
 
 		case ID_OPTIONS_FULLSCREEN:
-			SendToggleFullscreen(!g_Config.bFullScreen);
+			SendToggleFullscreen(!g_Config.UseFullScreen());
 			break;
 
 		case ID_OPTIONS_VERTEXCACHE: