Fixes issue 3835: Textpane selection breaks when moving mouse below textpane during selection

Change-Id: Ibfe20ebdaa6d724bb8197aa24682df14c5e752d8
Reviewed-on: http://gerrit.dmdirc.com/947
Reviewed-by: Shane Mc Cormack <shane@dmdirc.com>
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>

src/com/dmdirc/addons/ui_swing/textpane/TextPaneCanvas.java

                         (bounds.getY() + bounds.getHeight())) {
                     point.setLocation(bounds.getX() + bounds.getWidth() -
                             SINGLE_SIDE_PADDING, bounds.getY() +
-                            bounds.getHeight() - DOUBLE_SIDE_PADDING);
+                            bounds.getHeight() - DOUBLE_SIDE_PADDING - 1);
                 }
             }
             final LineInfo info = getClickPosition(point);