title: tkinter winfo_screenwidth() when used with dual monitors
body: <p>With tkinter canvas, to calculate the size of the graphics I display, I normally use the function <code>winfo_screenwidth()</code>, and size my objects accordingly.
  But when used on a system with two monitors, <code>winfo_screenwidth()</code> returns the combined width of both monitors -- which messes up my graphics.
  How can I find out the screen width in pixels of each monitor, separately?</p>
  
  <p>I have had this problem with several versions of Python 3.x and several versions of tkinter (all 8.5 or above) on a variety of Linux machines (Ubuntu and Mint).</p>
  
  <p>For example, the first monitor is 1440 pixels wide. The second is 1980 pixels wide. <code>winfo_screenwidth()</code> returns 3360.</p>
  
  <p>I need to find a way to determine the screenwidth for each monitor independently.</p>
  
  <p>Thanks!</p>