Windows: Handle fullscreen message consistently.

This handles it the same way as SDL, etc. so that the new button on the
main screen works again.

Windows/MainWindow.cpp

@@ -132,6 +132,7 @@ namespace MainWindow
 	static bool inFullscreenResize = false;
 	static bool inResizeMove = false;
 	static bool hasFocus = true;
+	static bool g_isFullscreen = false;
 
 	// gross hack
 	bool noFocusPause = false;	// TOGGLE_PAUSE state to override pause on lost focus
@@ -355,6 +356,7 @@ namespace MainWindow
 		::SetMenu(hWnd, goingFullscreen ? NULL : menu);
 
 		g_Config.bFullScreen = goingFullscreen;
+		g_isFullscreen = goingFullscreen;
 
 		g_IgnoreWM_SIZE = false;
 
@@ -1029,4 +1031,8 @@ namespace MainWindow
 		PostMessage(hwndMain, WM_USER_TOGGLE_FULLSCREEN, fullscreen, 0);
 	}
 
+	bool IsFullscreen() {
+		return g_isFullscreen;
+	}
+
 }  // namespace