bug#11464: 24.1.50; pos-visible-in-window-p returns a false positive with bidi text

[Eli Zaretskii]

> From: Michael Welsh Duggan <mwd@cert.org>
> Cc: Ari Roponen <ari.roponen@gmail.com>, 11464@debbugs.gnu.org
> Date: Thu, 17 May 2012 13:56:45 -0400
> 
>   else if (top_y < it.last_visible_y) ==> if (255 < 256) ==> if (TRUE)
> 
> hence, visible_p _would_ be set to 1 by the else if clause.

And will never be reset to zero, because 27 <= 256 => FALSE.  And yet
Ari said that he needed to change the condition to <=.