vncviewer: Fix scrollbar visibility

vncviewer: Fix scrollbar visibility

Have a window that is sized to the remote screen.  Now shrink the window
vertically, making it shorter than the remote screen in one axis. The
vertical scrollbar appears. However, the horizontal scrollbar does not
appear, despite the rightmost ~24 pixels (Fl::scrollbar_size()) being
hidden by the vertical scroll bar.

Fix that.

For clarity, move the fullscreen checks into a separate `if` statement,
rather than keeping the size and fullscreen checks together.

I think the comment does a decent job of explaining and justifying the
check's logic, but if you require further convincing, perhaps this
alternate explanation will help:

  The check for the X-axis is

      if ((w() - (h() < viewport->h() ? Fl::scrollbar_size() : 0) < viewport->w())

  To be a bit more verbose and repetitive, we can split that ternary in to
  two separate checks, and add some comments:

      if (
             (w() -  < viewport->w()) // X needs a scrollbar
          ||
             ( (w() - Fl::scrollbar_size() < viewport->w()) && (h() < viewport->h()) )
             //( X doesn't need a scrollbar unless Y does ) && (  Y does need one  ) )
         )

  Within the "Y does need one" check, we don't need to worry about the
  case where `h() - Fl::scrollbar_size() < viewport-h()` is true,
  because if both axes are saying "I don't need a scrollbar unless
  you do", then neither needs one.