Fix mouse grabbing and enable it in windowed mode

src/SDLSystem.cpp

 	if (mFullscreen)
 	{
 		flags |= SDL_FULLSCREEN;
-        SDL_ShowCursor(SDL_DISABLE);
 	}
 
+    SDL_ShowCursor(SDL_DISABLE);
+    setGrabMouse(true); // Always grab mouse!
+
 	SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 5);
 	SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 5);
 	SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 5);