Fixes issue 3640: Desktop pane background doesn't update when scale option changed

Change-Id: I4c889e4351907867b5d068576974bb4b309000ac
Reviewed-on: http://gerrit.dmdirc.com/658
Reviewed-by: Shane Mc Cormack <shane@dmdirc.com>
Automatic-Compile: Gregory Holmes <greg@dmdirc.com>

src/com/dmdirc/addons/ui_swing/components/desktopPane/DMDircDesktopPane.java

         } catch (IllegalArgumentException ex) {
             backgroundOption = BackgroundOption.CENTER;
         }
+        repaint();
     }
 
     /** {@inheritDoc} */