title: How to fade out a Bitmap with gtkmm?
body: <p>I use </p>
  
  <pre><code>Glib::RefPtr&lt;Gdk::Pixbuf&gt; image = Gdk::Pixbuf::create_from_file(sBitmapName);
  Gdk::Cairo::set_source_pixbuf(cr, image, x, y);
  </code></pre>
  
  <p>to load and display a bitmap in a cairo context.</p>
  
  <p>Is it possible to manipulate the pixels in the pixbuf once it is loaded?
  I would like to fade out this image by decreasing the the alpha value of each pixel.</p>
  
  <p>Or is there another possibility to fade out a pixmap?</p>