Fix double mouse motion in OS X

Simply ignore the event handler for Windowed mode in fullscreen.

src/video/cocoa/SDL_cocoawindow.m

@@ -232,6 +232,9 @@ - (void)mouseMoved:(NSEvent *)theEvent
     SDL_Window *window = _data->window;
     NSPoint point;
 
+    if (window->flags & SDL_WINDOW_FULLSCREEN)
+        return;
+
     point = [theEvent locationInWindow];
     point.y = window->h - point.y;
     if ( point.x < 0 || point.x >= window->w ||