bug#16647: Imprecisions with window-resizing cursors

[martin rudalics]

I now tried to address the following issues reported here:

> On current trunk version, from emacs -Q
>
> 1)
>
> (progn
>   (scroll-bar-mode -1)
>   (split-window-right)
>   )
>
> Slowly move the mouse from left to right across the vertical line splitting the 
windows. Once the mouse goes past the black line, it's still shown as <=>, 
although clicking-and-dragging on that area won't have any effect.

Confirmed.  The reason was that a mouse glyph could cross window
borders.  Should be fixed now.

> 2)
>
> C-x 2
>
> Move the cursor from top to bottom, across the mode-line. Once the cursor enters the 
mode-line, it won't change to a "resize" cursor, although it's already possible to 
drag.

Confirmed (on a Gtk build).  The reason was that a mouse glyph from the
text area could extend into the mode line.  Should be fixed now.

martin