title: Scan for keyboard input without pausing
body: <p>I'm looking for a way to have R periodically check for keyboard input from the graphics window and then continue execution if there is none. There is a <code>repeat</code> loop that continuously updates graphics in the window, and I would like it to react to some keyboard input, but not <b>wait</b> for input. However, I'm only aware of commands like <code>scan</code> or <code>getGraphicsEvent</code> that pause execution until input is received.</p>
  
  <p>Is there a way to, basically, scan the keyboard buffer and just continue execution if it is empty?</p>